Introduction
============

This directory contains Device Tree overlays. Device Tree makes it possible
to support many hardware configurations with a single kernel and without the
need to explicitly load or blacklist kernel modules. Note that this isn't a
"pure" Device Tree configuration (c.f. MACH_BCM2835) - some on-board devices
are still configured by the board support code, but the intention is to
eventually reach that goal.

On Raspberry Pi, Device Tree usage is controlled from /boot/config.txt. By
default, the Raspberry Pi kernel boots with device tree enabled. You can
completely disable DT usage (for now) by adding:

    device_tree=

to your config.txt, which should cause your Pi to revert to the old way of
doing things after a reboot.

In /boot you will find a .dtb for each base platform. This describes the
hardware that is part of the Raspberry Pi board. The loader (start.elf and its
siblings) selects the .dtb file appropriate for the platform by name, and reads
it into memory. At this point, all of the optional interfaces (i2c, i2s, spi)
are disabled, but they can be enabled using Device Tree parameters:

    dtparam=i2c=on,i2s=on,spi=on

However, this shouldn't be necessary in many use cases because loading an
overlay that requires one of those interfaces will cause it to be enabled
automatically, and it is advisable to only enable interfaces if they are
needed.

Configuring additional, optional hardware is done using Device Tree overlays
(see below).

raspi-config
============

The Advanced Options section of the raspi-config utility can enable and disable
Device Tree use, as well as toggling the I2C and SPI interfaces. Note that it
is possible to both enable an interface and blacklist the driver, if for some
reason you should want to defer the loading.

Modules
=======

As well as describing the hardware, Device Tree also gives enough information
to allow suitable driver modules to be located and loaded, with the corollary
that unneeded modules are not loaded. As a result it should be possible to
remove lines from /etc/modules, and /etc/modprobe.d/raspi-blacklist.conf can
have its contents deleted (or commented out).

Using Overlays
==============

Overlays are loaded using the "dtoverlay" directive. As an example, consider
the popular lirc-rpi module, the Linux Infrared Remote Control driver. In the
pre-DT world this would be loaded from /etc/modules, with an explicit
"modprobe lirc-rpi" command, or programmatically by lircd. With DT enabled,
this becomes a line in config.txt:

    dtoverlay=lirc-rpi

This causes the file /boot/overlays/lirc-rpi.dtbo to be loaded. By
default it will use GPIOs 17 (out) and 18 (in), but this can be modified using
DT parameters:

    dtoverlay=lirc-rpi,gpio_out_pin=17,gpio_in_pin=13

Parameters always have default values, although in some cases (e.g. "w1-gpio")
it is necessary to provided multiple overlays in order to get the desired
behaviour. See the list of overlays below for a description of the parameters
and their defaults.

The Overlay and Parameter Reference
===================================

N.B. When editing this file, please preserve the indentation levels to make it
simple to parse programmatically. NO HARD TABS.


Name:   <The base DTB>
Info:   Configures the base Raspberry Pi hardware
Load:   <loaded automatically>
Params:
        audio                   Set to "on" to enable the onboard ALSA audio
                                interface (default "off")

        i2c_arm                 Set to "on" to enable the ARM's i2c interface
                                (default "off")

        i2c_vc                  Set to "on" to enable the i2c interface
                                usually reserved for the VideoCore processor
                                (default "off")

        i2c                     An alias for i2c_arm

        i2c_arm_baudrate        Set the baudrate of the ARM's i2c interface
                                (default "100000")

        i2c_vc_baudrate         Set the baudrate of the VideoCore i2c interface
                                (default "100000")

        i2c_baudrate            An alias for i2c_arm_baudrate

        i2s                     Set to "on" to enable the i2s interface
                                (default "off")

        spi                     Set to "on" to enable the spi interfaces
                                (default "off")

        random                  Set to "on" to enable the hardware random
                                number generator (default "on")

        sd_overclock            Clock (in MHz) to use when the MMC framework
                                requests 50MHz

        sd_force_pio            Disable DMA support for SD driver (default off)

        sd_pio_limit            Number of blocks above which to use DMA for
                                SD card (default 1)

        sd_debug                Enable debug output from SD driver (default off)

        uart0                   Set to "off" to disable uart0 (default "on")

        uart1                   Set to "on" or "off" to enable or disable uart1
                                (default varies)

        watchdog                Set to "on" to enable the hardware watchdog
                                (default "off")

        act_led_trigger         Choose which activity the LED tracks.
                                Use "heartbeat" for a nice load indicator.
                                (default "mmc")

        act_led_activelow       Set to "on" to invert the sense of the LED
                                (default "off")
                                N.B. For Pi3 see pi3-act-led overlay.

        act_led_gpio            Set which GPIO to use for the activity LED
                                (in case you want to connect it to an external
                                device)
                                (default "16" on a non-Plus board, "47" on a
                                Plus or Pi 2)
                                N.B. For Pi3 see pi3-act-led overlay.

        pwr_led_trigger
        pwr_led_activelow
        pwr_led_gpio
                                As for act_led_*, but using the PWR LED.
                                Not available on Model A/B boards.

        N.B. It is recommended to only enable those interfaces that are needed.
        Leaving all interfaces enabled can lead to unwanted behaviour (i2c_vc
        interfering with Pi Camera, I2S and SPI hogging GPIO pins, etc.)
        Note also that i2c, i2c_arm and i2c_vc are aliases for the physical
        interfaces i2c0 and i2c1. Use of the numeric variants is still possible
        but deprecated because the ARM/VC assignments differ between board
        revisions. The same board-specific mapping applies to i2c_baudrate,
        and the other i2c baudrate parameters.


Name:   adau1977-adc
Info:   Overlay for activation of ADAU1977 ADC codec over I2C for control
        and I2S for data.
Load:   dtoverlay=adau1977-adc
Params: <None>


Name:   ads1015
Info:   Overlay for activation of Texas Instruments ADS1015 ADC over I2C
Load:   dtoverlay=ads1015,<param>=<val>
Params: addr                    I2C bus address of device. Set based on how the
                                addr pin is wired. (default=0x48 assumes addr
                                is pulled to GND)
        cha_enable              Enable virtual channel a. (default=true)
        cha_cfg                 Set the configuration for virtual channel a.
                                (default=4 configures this channel for the
                                voltage at A0 with respect to GND)
        cha_datarate            Set the datarate (samples/sec) for this channel.
                                (default=4 sets 1600 sps)
        cha_gain                Set the gain of the Programmable Gain
                                Amplifier for this channel. (default=2 sets the
                                full scale of the channel to 2.048 Volts)

        Channel (ch) parameters can be set for each enabled channel.
        A maximum of 4 channels can be enabled (letters a thru d).
        For more information refer to the device datasheet at:
        http://www.ti.com/lit/ds/symlink/ads1015.pdf


Name:   ads7846
Info:   ADS7846 Touch controller
Load:   dtoverlay=ads7846,<param>=<val>
Params: cs                      SPI bus Chip Select (default 1)
        speed                   SPI bus speed (default 2MHz, max 3.25MHz)
        penirq                  GPIO used for PENIRQ. REQUIRED
        penirq_pull             Set GPIO pull (default 0=none, 2=pullup)
        swapxy                  Swap x and y axis
        xmin                    Minimum value on the X axis (default 0)
        ymin                    Minimum value on the Y axis (default 0)
        xmax                    Maximum value on the X axis (default 4095)
        ymax                    Maximum value on the Y axis (default 4095)
        pmin                    Minimum reported pressure value (default 0)
        pmax                    Maximum reported pressure value (default 65535)
        xohms                   Touchpanel sensitivity (X-plate resistance)
                                (default 400)

        penirq is required and usually xohms (60-100) has to be set as well.
        Apart from that, pmax (255) and swapxy are also common.
        The rest of the calibration can be done with xinput-calibrator.
        See: github.com/notro/fbtft/wiki/FBTFT-on-Raspian
        Device Tree binding document:
        www.kernel.org/doc/Documentation/devicetree/bindings/input/ads7846.txt


Name:   akkordion-iqdacplus
Info:   Configures the Digital Dreamtime Akkordion Music Player (based on the
        OEM IQAudIO DAC+ or DAC Zero module).
Load:   dtoverlay=akkordion-iqdacplus,<param>=<val>
Params: 24db_digital_gain       Allow gain to be applied via the PCM512x codec
                                Digital volume control. Enable with
                                dtoverlay=akkordion-iqdacplus,24db_digital_gain
                                (The default behaviour is that the Digital
                                volume control is limited to a maximum of
                                0dB. ie. it can attenuate but not provide
                                gain. For most users, this will be desired
                                as it will prevent clipping. By appending
                                the 24db_digital_gain parameter, the Digital
                                volume control will allow up to 24dB of
                                gain. If this parameter is enabled, it is the
                                responsibility of the user to ensure that
                                the Digital volume control is set to a value
                                that does not result in clipping/distortion!)


Name:   allo-piano-dac-pcm512x-audio
Info:   Configures the Allo Piano DAC (2.0/2.1) audio cards.
        (NB. This initial support is for 2.0 channel audio ONLY! ie. stereo.
        The subwoofer outputs on the Piano 2.1 are not currently supported!)
Load:   dtoverlay=allo-piano-dac-pcm512x-audio,<param>
Params: 24db_digital_gain       Allow gain to be applied via the PCM512x codec
                                Digital volume control.
                                (The default behaviour is that the Digital
                                volume control is limited to a maximum of
                                0dB. ie. it can attenuate but not provide
                                gain. For most users, this will be desired
                                as it will prevent clipping. By appending
                                the 24db_digital_gain parameter, the Digital
                                volume control will allow up to 24dB of
                                gain. If this parameter is enabled, it is the
                                responsibility of the user to ensure that
                                the Digital volume control is set to a value
                                that does not result in clipping/distortion!)


Name:   at86rf233
Info:   Configures the Atmel AT86RF233 802.15.4 low-power WPAN transceiver,
        connected to spi0.0
Load:   dtoverlay=at86rf233,<param>=<val>
Params: interrupt               GPIO used for INT (default 23)
        reset                   GPIO used for Reset (default 24)
        sleep                   GPIO used for Sleep (default 25)
        speed                   SPI bus speed in Hz (default 3000000)
        trim                    Fine tuning of the internal capacitance
                                arrays (0=+0pF, 15=+4.5pF, default 15)


Name:   audioinjector-wm8731-audio
Info:   Configures the audioinjector.net audio add on soundcard
Load:   dtoverlay=audioinjector-wm8731-audio
Params: <None>


Name:   audremap
Info:   Switches PWM sound output to pins 12 (Right) & 13 (Left)
Load:   dtoverlay=audremap,<param>=<val>
Params: swap_lr                 Reverse the channel allocation, which will also
                                swap the audio jack outputs (default off)
        enable_jack             Don't switch off the audio jack output
                                (default off)


Name:   bmp085_i2c-sensor
Info:   This overlay is now deprecated - see i2c-sensor
Load:   dtoverlay=bmp085_i2c-sensor
Params: <None>


Name:   dht11
Info:   Overlay for the DHT11/DHT21/DHT22 humidity/temperature sensors
        Also sometimes found with the part number(s) AM230x.
Load:   dtoverlay=dht11,<param>=<val>
Params: gpiopin                 GPIO connected to the sensor's DATA output.
                                (default 4)


Name:   dionaudio-loco
Info:   Configures the Dion Audio LOCO DAC-AMP
Load:   dtoverlay=dionaudio-loco
Params: <None>


Name:   dpi18
Info:   Overlay for a generic 18-bit DPI display
        This uses GPIOs 0-21 (so no I2C, uart etc.), and activates the output
        2-3 seconds after the kernel has started.
Load:   dtoverlay=dpi18
Params: <None>


Name:   dpi24
Info:   Overlay for a generic 24-bit DPI display
        This uses GPIOs 0-27 (so no I2C, uart etc.), and activates the output
        2-3 seconds after the kernel has started.
Load:   dtoverlay=dpi24
Params: <None>


Name:   dwc-otg
Info:   Selects the dwc_otg USB controller driver which has fiq support. This
        is the default on all except the Pi Zero which defaults to dwc2.
Load:   dtoverlay=dwc-otg
Params: <None>


Name:   dwc2
Info:   Selects the dwc2 USB controller driver
Load:   dtoverlay=dwc2,<param>=<val>
Params: dr_mode                 Dual role mode: "host", "peripheral" or "otg"

        g-rx-fifo-size          Size of rx fifo size in gadget mode

        g-np-tx-fifo-size       Size of non-periodic tx fifo size in gadget
                                mode

        g-tx-fifo-size          Size of periodic tx fifo per endpoint
                                (except ep0) in gadget mode


[ The ds1307-rtc overlay has been deleted. See i2c-rtc. ]


Name:   enc28j60
Info:   Overlay for the Microchip ENC28J60 Ethernet Controller on SPI0
Load:   dtoverlay=enc28j60,<param>=<val>
Params: int_pin                 GPIO used for INT (default 25)

        speed                   SPI bus speed (default 12000000)


Name:   enc28j60-spi2
Info:   Overlay for the Microchip ENC28J60 Ethernet Controller on SPI2
Load:   dtoverlay=enc28j60-spi2,<param>=<val>
Params: int_pin                 GPIO used for INT (default 39)

        speed                   SPI bus speed (default 12000000)


Name:   gpio-ir
Info:   Use GPIO pin as rc-core style infrared receiver input. The rc-core-
        based gpio_ir_recv driver maps received keys directly to a
        /dev/input/event* device, all decoding is done by the kernel - LIRC is
        not required! The key mapping and other decoding parameters can be
        configured by "ir-keytable" tool.
Load:   dtoverlay=gpio-ir,<param>=<val>
Params: gpio_pin                Input pin number. Default is 18.

        gpio_pull               Desired pull-up/down state (off, down, up)
                                Default is "down".

        rc-map-name             Default rc keymap (can also be changed by
                                ir-keytable), defaults to "rc-rc6-mce"


Name:   gpio-poweroff
Info:   Drives a GPIO high or low on poweroff (including halt)
Load:   dtoverlay=gpio-poweroff,<param>=<val>
Params: gpiopin                 GPIO for signalling (default 26)

        active_low              Set if the power control device requires a
                                high->low transition to trigger a power-down.
                                Note that this will require the support of a
                                custom dt-blob.bin to prevent a power-down
                                during the boot process, and that a reboot
                                will also cause the pin to go low.


Name:   hifiberry-amp
Info:   Configures the HifiBerry Amp and Amp+ audio cards
Load:   dtoverlay=hifiberry-amp
Params: <None>


Name:   hifiberry-dac
Info:   Configures the HifiBerry DAC audio card
Load:   dtoverlay=hifiberry-dac
Params: <None>


Name:   hifiberry-dacplus
Info:   Configures the HifiBerry DAC+ audio card
Load:   dtoverlay=hifiberry-dacplus,<param>=<val>
Params: 24db_digital_gain       Allow gain to be applied via the PCM512x codec
                                Digital volume control. Enable with
                                "dtoverlay=hifiberry-dacplus,24db_digital_gain"
                                (The default behaviour is that the Digital
                                volume control is limited to a maximum of
                                0dB. ie. it can attenuate but not provide
                                gain. For most users, this will be desired
                                as it will prevent clipping. By appending
                                the 24dB_digital_gain parameter, the Digital
                                volume control will allow up to 24dB of
                                gain. If this parameter is enabled, it is the
                                responsibility of the user to ensure that
                                the Digital volume control is set to a value
                                that does not result in clipping/distortion!)
        slave                   Force DAC+ Pro into slave mode, using Pi as
                                master for bit clock and frame clock.


Name:   hifiberry-digi
Info:   Configures the HifiBerry Digi and Digi+ audio card
Load:   dtoverlay=hifiberry-digi
Params: <None>


Name:   hifiberry-digi-pro
Info:   Configures the HifiBerry Digi+ Pro audio card
Load:   dtoverlay=hifiberry-digi-pro
Params: <None>


Name:   hy28a
Info:   HY28A - 2.8" TFT LCD Display Module by HAOYU Electronics
        Default values match Texy's display shield
Load:   dtoverlay=hy28a,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        debug                   Debug output level {0-7}

        xohms                   Touchpanel sensitivity (X-plate resistance)

        resetgpio               GPIO used to reset controller

        ledgpio                 GPIO used to control backlight


Name:   hy28b
Info:   HY28B - 2.8" TFT LCD Display Module by HAOYU Electronics
        Default values match Texy's display shield
Load:   dtoverlay=hy28b,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        debug                   Debug output level {0-7}

        xohms                   Touchpanel sensitivity (X-plate resistance)

        resetgpio               GPIO used to reset controller

        ledgpio                 GPIO used to control backlight


Name:   i2c-gpio
Info:   Adds support for software i2c controller on gpio pins
Load:   dtoverlay=i2c-gpio,<param>=<val>
Params: i2c_gpio_sda            GPIO used for I2C data (default "23")

        i2c_gpio_scl            GPIO used for I2C clock (default "24")

        i2c_gpio_delay_us       Clock delay in microseconds
                                (default "2" = ~100kHz)


Name:   i2c-mux
Info:   Adds support for a number of I2C bus multiplexers on i2c_arm
Load:   dtoverlay=i2c-mux,<param>=<val>
Params: pca9542                 Select the NXP PCA9542 device

        pca9545                 Select the NXP PCA9545 device

        pca9548                 Select the NXP PCA9548 device

        addr                    Change I2C address of the device (default 0x70)


[ The i2c-mux-pca9548a overlay has been deleted. See i2c-mux. ]


Name:   i2c-pwm-pca9685a
Info:   Adds support for an NXP PCA9685A I2C PWM controller on i2c_arm
Load:   dtoverlay=i2c-pwm-pca9685a,<param>=<val>
Params: addr                    I2C address of PCA9685A (default 0x40)


Name:   i2c-rtc
Info:   Adds support for a number of I2C Real Time Clock devices
Load:   dtoverlay=i2c-rtc,<param>=<val>
Params: abx80x                  Select one of the ABx80x family:
                                  AB0801, AB0803, AB0804, AB0805,
                                  AB1801, AB1803, AB1804, AB1805

        ds1307                  Select the DS1307 device

        ds1339                  Select the DS1339 device

        ds3231                  Select the DS3231 device

        mcp7940x                Select the MCP7940x device

        mcp7941x                Select the MCP7941x device

        pcf2127                 Select the PCF2127 device

        pcf8523                 Select the PCF8523 device

        pcf8563                 Select the PCF8563 device

        trickle-diode-type      Diode type for trickle charge - "standard" or
                                "schottky" (ABx80x only)

        trickle-resistor-ohms   Resistor value for trickle charge (DS1339,
                                ABx80x)

        wakeup-source           Specify that the RTC can be used as a wakeup
                                source


Name:   i2c-sensor
Info:   Adds support for a number of I2C barometric pressure and temperature
        sensors on i2c_arm
Load:   dtoverlay=i2c-sensor,<param>=<val>
Params: bmp085                  Select the Bosch sensortronic BMP085

        bmp280                  Select the Bosch sensortronic BMP280

        si7020                  Select the Silicon Labs Si7013/20/21 humidity/
                                temperature sensor


Name:   i2c0-bcm2708
Info:   Enable the i2c_bcm2708 driver for the i2c0 bus. Not all pin combinations
        are usable on all platforms.
Load:   dtoverlay=i2c0-bcm2708,<param>=<val>
Params: sda0_pin                GPIO pin for SDA0 (deprecated - use pins_*)
        scl0_pin                GPIO pin for SCL0 (deprecated - use pins_*)
        pins_0_1                Use pins 0 and 1 (default)
        pins_28_29              Use pins 28 and 29
        pins_44_45              Use pins 44 and 45
        pins_46_47              Use pins 46 and 47


Name:   i2c1-bcm2708
Info:   Enable the i2c_bcm2708 driver for the i2c1 bus
Load:   dtoverlay=i2c1-bcm2708,<param>=<val>
Params: sda1_pin                GPIO pin for SDA1 (2 or 44 - default 2)
        scl1_pin                GPIO pin for SCL1 (3 or 45 - default 3)
        pin_func                Alternative pin function (4 (alt0), 6 (alt2) -
                                default 4)


Name:   i2s-gpio28-31
Info:   move I2S function block to GPIO 28 to 31
Load:   dtoverlay=i2s-gpio28-31
Params: <None>


Name:   i2s-mmap
Info:   Enables mmap support in the bcm2708-i2s driver
Load:   dtoverlay=i2s-mmap
Params: <None>


Name:   iqaudio-dac
Info:   Configures the IQaudio DAC audio card
Load:   dtoverlay=iqaudio-dac,<param>
Params: 24db_digital_gain       Allow gain to be applied via the PCM512x codec
                                Digital volume control. Enable with
                                "dtoverlay=iqaudio-dac,24db_digital_gain"
                                (The default behaviour is that the Digital
                                volume control is limited to a maximum of
                                0dB. ie. it can attenuate but not provide
                                gain. For most users, this will be desired
                                as it will prevent clipping. By appending
                                the 24db_digital_gain parameter, the Digital
                                volume control will allow up to 24dB of
                                gain. If this parameter is enabled, it is the
                                responsibility of the user to ensure that
                                the Digital volume control is set to a value
                                that does not result in clipping/distortion!)


Name:   iqaudio-dacplus
Info:   Configures the IQaudio DAC+ audio card
Load:   dtoverlay=iqaudio-dacplus,<param>=<val>
Params: 24db_digital_gain       Allow gain to be applied via the PCM512x codec
                                Digital volume control. Enable with
                                "dtoverlay=iqaudio-dacplus,24db_digital_gain"
                                (The default behaviour is that the Digital
                                volume control is limited to a maximum of
                                0dB. ie. it can attenuate but not provide
                                gain. For most users, this will be desired
                                as it will prevent clipping. By appending
                                the 24db_digital_gain parameter, the Digital
                                volume control will allow up to 24dB of
                                gain. If this parameter is enabled, it is the
                                responsibility of the user to ensure that
                                the Digital volume control is set to a value
                                that does not result in clipping/distortion!)
        auto_mute_amp           If specified, unmute/mute the IQaudIO amp when
                                starting/stopping audio playback.
        unmute_amp              If specified, unmute the IQaudIO amp once when
                                the DAC driver module loads.


Name:   iqaudio-digi-wm8804-audio
Info:   Configures the IQAudIO Digi WM8804 audio card
Load:   dtoverlay=iqaudio-digi-wm8804-audio,<param>=<val>
Params: card_name               Override the default, "IQAudIODigi", card name.
        dai_name                Override the default, "IQAudIO Digi", dai name.
        dai_stream_name         Override the default, "IQAudIO Digi HiFi",
                                dai stream name.


Name:   justboom-dac
Info:   Configures the JustBoom DAC HAT, Amp HAT, DAC Zero and Amp Zero audio
        cards
Load:   dtoverlay=justboom-dac,<param>=<val>
Params: 24db_digital_gain       Allow gain to be applied via the PCM512x codec
                                Digital volume control. Enable with
                                "dtoverlay=justboom-dac,24db_digital_gain"
                                (The default behaviour is that the Digital
                                volume control is limited to a maximum of
                                0dB. ie. it can attenuate but not provide
                                gain. For most users, this will be desired
                                as it will prevent clipping. By appending
                                the 24dB_digital_gain parameter, the Digital
                                volume control will allow up to 24dB of
                                gain. If this parameter is enabled, it is the
                                responsibility of the user to ensure that
                                the Digital volume control is set to a value
                                that does not result in clipping/distortion!)


Name:   justboom-digi
Info:   Configures the JustBoom Digi HAT and Digi Zero audio cards
Load:   dtoverlay=justboom-digi
Params: <None>


Name:   lirc-rpi
Info:   Configures lirc-rpi (Linux Infrared Remote Control for Raspberry Pi)
        Consult the module documentation for more details.
Load:   dtoverlay=lirc-rpi,<param>=<val>
Params: gpio_out_pin            GPIO for output (default "17")

        gpio_in_pin             GPIO for input (default "18")

        gpio_in_pull            Pull up/down/off on the input pin
                                (default "down")

        sense                   Override the IR receive auto-detection logic:
                                 "0" = force active-high
                                 "1" = force active-low
                                 "-1" = use auto-detection
                                (default "-1")

        softcarrier             Turn the software carrier "on" or "off"
                                (default "on")

        invert                  "on" = invert the output pin (default "off")

        debug                   "on" = enable additional debug messages
                                (default "off")


Name:   mcp23017
Info:   Configures the MCP23017 I2C GPIO expander
Load:   dtoverlay=mcp23017,<param>=<val>
Params: gpiopin                 Gpio pin connected to the INTA output of the
                                MCP23017 (default: 4)

        addr                    I2C address of the MCP23017 (default: 0x20)


Name:   mcp23s17
Info:   Configures the MCP23S08/17 SPI GPIO expanders.
        If devices are present on SPI1 or SPI2, those interfaces must be enabled
        with one of the spi1-1/2/3cs and/or spi2-1/2/3cs overlays.
        If interrupts are enabled for a device on a given CS# on a SPI bus, that
        device must be the only one present on that SPI bus/CS#.
Load:   dtoverlay=mcp23s17,<param>=<val>
Params: s08-spi<n>-<m>-present  4-bit integer, bitmap indicating MCP23S08
                                devices present on SPI<n>, CS#<m>

        s17-spi<n>-<m>-present  8-bit integer, bitmap indicating MCP23S17
                                devices present on SPI<n>, CS#<m>

        s08-spi<n>-<m>-int-gpio integer, enables interrupts on a single
                                MCP23S08 device on SPI<n>, CS#<m>, specifies
                                the GPIO pin to which INT output of MCP23S08
                                is connected.

        s17-spi<n>-<m>-int-gpio integer, enables mirrored interrupts on a
                                single MCP23S17 device on SPI<n>, CS#<m>,
                                specifies the GPIO pin to which either INTA
                                or INTB output of MCP23S17 is connected.


Name:   mcp2515-can0
Info:   Configures the MCP2515 CAN controller on spi0.0
Load:   dtoverlay=mcp2515-can0,<param>=<val>
Params: oscillator              Clock frequency for the CAN controller (Hz)

        spimaxfrequency         Maximum SPI frequence (Hz)

        interrupt               GPIO for interrupt signal


Name:   mcp2515-can1
Info:   Configures the MCP2515 CAN controller on spi0.1
Load:   dtoverlay=mcp2515-can1,<param>=<val>
Params: oscillator              Clock frequency for the CAN controller (Hz)

        spimaxfrequency         Maximum SPI frequence (Hz)

        interrupt               GPIO for interrupt signal


Name:   mcp3008
Info:   Configures MCP3008 A/D converters
        For devices on spi1 or spi2, the interfaces should be enabled
        with one of the spi1-1/2/3cs and/or spi2-1/2/3cs overlays.
Load:   dtoverlay=mcp3008,<param>[=<val>]
Params: spi<n>-<m>-present      boolean, configure device at spi<n>, cs<m>
        spi<n>-<m>-speed        integer, set the spi bus speed for this device


Name:   mmc
Info:   Selects the bcm2835-mmc SD/MMC driver, optionally with overclock
Load:   dtoverlay=mmc,<param>=<val>
Params: overclock_50            Clock (in MHz) to use when the MMC framework
                                requests 50MHz


Name:   mz61581
Info:   MZ61581 display by Tontec
Load:   dtoverlay=mz61581,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        txbuflen                Transmit buffer length (default 32768)

        debug                   Debug output level {0-7}

        xohms                   Touchpanel sensitivity (X-plate resistance)


[ The pcf2127-rtc overlay has been deleted. See i2c-rtc. ]


[ The pcf8523-rtc overlay has been deleted. See i2c-rtc. ]


[ The pcf8563-rtc overlay has been deleted. See i2c-rtc. ]


Name:   pi3-act-led
Info:   Pi3 uses a GPIO expander to drive the LEDs which can only be accessed
        from the VPU. There is a special driver for this with a separate DT
        node, which has the unfortunate consequence of breaking the
        act_led_gpio and act_led_activelow dtparams.
        This overlay changes the GPIO controller back to the standard one and
        restores the dtparams.
Load:   dtoverlay=pi3-act-led,<param>=<val>
Params: activelow               Set to "on" to invert the sense of the LED
                                (default "off")

        gpio                    Set which GPIO to use for the activity LED
                                (in case you want to connect it to an external
                                device)
                                REQUIRED


Name:   pi3-disable-bt
Info:   Disable Pi3 Bluetooth and restore UART0/ttyAMA0 over GPIOs 14 & 15
        N.B. To disable the systemd service that initialises the modem so it
        doesn't use the UART, use 'sudo systemctl disable hciuart'.
Load:   dtoverlay=pi3-disable-bt
Params: <None>


Name:   pi3-disable-wifi
Info:   Disable Pi3 onboard WiFi
Load:   dtoverlay=pi3-disable-wifi
Params: <None>


Name:   pi3-miniuart-bt
Info:   Switch Pi3 Bluetooth function to use the mini-UART (ttyS0) and restore
        UART0/ttyAMA0 over GPIOs 14 & 15. Note that this may reduce the maximum
        usable baudrate.
        N.B. It is also necessary to edit /lib/systemd/system/hciuart.service
        and replace ttyAMA0 with ttyS0, unless you have a system with udev rules
        that create /dev/serial0 and /dev/serial1, in which case use
        /dev/serial1 instead because it will always be correct. Furthermore,
        you must also set core_freq=250 in config.txt or the miniuart will not
        work.
Load:   dtoverlay=pi3-miniuart-bt
Params: <None>


Name:   piscreen
Info:   PiScreen display by OzzMaker.com
Load:   dtoverlay=piscreen,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        debug                   Debug output level {0-7}

        xohms                   Touchpanel sensitivity (X-plate resistance)


Name:   piscreen2r
Info:   PiScreen 2 with resistive TP display by OzzMaker.com
Load:   dtoverlay=piscreen2r,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        debug                   Debug output level {0-7}

        xohms                   Touchpanel sensitivity (X-plate resistance)


Name:   pisound
Info:   Configures the Blokas Labs pisound card
Load:   dtoverlay=pisound
Params: <None>


Name:   pitft22
Info:   Adafruit PiTFT 2.2" screen
Load:   dtoverlay=pitft22,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        debug                   Debug output level {0-7}


Name:   pitft28-capacitive
Info:   Adafruit PiTFT 2.8" capacitive touch screen
Load:   dtoverlay=pitft28-capacitive,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        debug                   Debug output level {0-7}

        touch-sizex             Touchscreen size x (default 240)

        touch-sizey             Touchscreen size y (default 320)

        touch-invx              Touchscreen inverted x axis

        touch-invy              Touchscreen inverted y axis

        touch-swapxy            Touchscreen swapped x y axis


Name:   pitft28-resistive
Info:   Adafruit PiTFT 2.8" resistive touch screen
Load:   dtoverlay=pitft28-resistive,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        debug                   Debug output level {0-7}


Name:   pitft35-resistive
Info:   Adafruit PiTFT 3.5" resistive touch screen
Load:   dtoverlay=pitft35-resistive,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        debug                   Debug output level {0-7}


Name:   pps-gpio
Info:   Configures the pps-gpio (pulse-per-second time signal via GPIO).
Load:   dtoverlay=pps-gpio,<param>=<val>
Params: gpiopin                 Input GPIO (default "18")
        assert_falling_edge     When present, assert is indicated by a falling
                                edge, rather than by a rising edge


Name:   pwm
Info:   Configures a single PWM channel
        Legal pin,function combinations for each channel:
          PWM0: 12,4(Alt0) 18,2(Alt5) 40,4(Alt0)            52,5(Alt1)
          PWM1: 13,4(Alt0) 19,2(Alt5) 41,4(Alt0) 45,4(Alt0) 53,5(Alt1)
        N.B.:
          1) Pin 18 is the only one available on all platforms, and
             it is the one used by the I2S audio interface.
             Pins 12 and 13 might be better choices on an A+, B+ or Pi2.
          2) The onboard analogue audio output uses both PWM channels.
          3) So be careful mixing audio and PWM.
          4) Currently the clock must have been enabled and configured
             by other means.
Load:   dtoverlay=pwm,<param>=<val>
Params: pin                     Output pin (default 18) - see table
        func                    Pin function (default 2 = Alt5) - see above
        clock                   PWM clock frequency (informational)


Name:   pwm-2chan
Info:   Configures both PWM channels
        Legal pin,function combinations for each channel:
          PWM0: 12,4(Alt0) 18,2(Alt5) 40,4(Alt0)            52,5(Alt1)
          PWM1: 13,4(Alt0) 19,2(Alt5) 41,4(Alt0) 45,4(Alt0) 53,5(Alt1)
        N.B.:
          1) Pin 18 is the only one available on all platforms, and
             it is the one used by the I2S audio interface.
             Pins 12 and 13 might be better choices on an A+, B+ or Pi2.
          2) The onboard analogue audio output uses both PWM channels.
          3) So be careful mixing audio and PWM.
          4) Currently the clock must have been enabled and configured
             by other means.
Load:   dtoverlay=pwm-2chan,<param>=<val>
Params: pin                     Output pin (default 18) - see table
        pin2                    Output pin for other channel (default 19)
        func                    Pin function (default 2 = Alt5) - see above
        func2                   Function for pin2 (default 2 = Alt5)
        clock                   PWM clock frequency (informational)


Name:   qca7000
Info:   I2SE's Evaluation Board for PLC Stamp micro
Load:   dtoverlay=qca7000,<param>=<val>
Params: int_pin                 GPIO pin for interrupt signal (default 23)

        speed                   SPI bus speed (default 12 MHz)


Name:   raspidac3
Info:   Configures the RaspiDAV Rev.3x audio card
Load:   dtoverlay=raspidac3
Params: <None>


Name:   rpi-backlight
Info:   Raspberry Pi official display backlight driver
Load:   dtoverlay=rpi-backlight
Params: <None>


Name:   rpi-dac
Info:   Configures the RPi DAC audio card
Load:   dtoverlay=rpi-dac
Params: <None>


Name:   rpi-display
Info:   RPi-Display - 2.8" Touch Display by Watterott
Load:   dtoverlay=rpi-display,<param>=<val>
Params: speed                   Display SPI bus speed
        rotate                  Display rotation {0,90,180,270}
        fps                     Delay between frame updates
        debug                   Debug output level {0-7}
        xohms                   Touchpanel sensitivity (X-plate resistance)
        swapxy                  Swap x and y axis


Name:   rpi-ft5406
Info:   Official Raspberry Pi display touchscreen
Load:   dtoverlay=rpi-ft5406
Params: <None>


Name:   rpi-proto
Info:   Configures the RPi Proto audio card
Load:   dtoverlay=rpi-proto
Params: <None>


Name:   rpi-sense
Info:   Raspberry Pi Sense HAT
Load:   dtoverlay=rpi-sense
Params: <None>


Name:   rra-digidac1-wm8741-audio
Info:   Configures the Red Rocks Audio DigiDAC1 soundcard
Load:   dtoverlay=rra-digidac1-wm8741-audio
Params: <None>


Name:   sc16is750-i2c
Info:   Overlay for the NXP SC16IS750 UART with I2C Interface
        Enables the chip on I2C1 at 0x48. To select another address,
        please refer to table 10 in reference manual.

Load:   dtoverlay=sc16is750-i2c,<param>=<val>
Params: int_pin                 GPIO used for IRQ (default 24)
        addr                    Address (default 0x48)


Name:   sc16is752-spi1
Info:   Overlay for the NXP SC16IS752 Dual UART with SPI Interface
        Enables the chip on SPI1.
        N.B.: spi1 is only accessible on devices with a 40pin header, eg:
              A+, B+, Zero and PI2 B; as well as the Compute Module.

Load:   dtoverlay=sc16is752-spi1,<param>=<val>
Params: int_pin                 GPIO used for IRQ (default 24)


Name:   sdhost
Info:   Selects the bcm2835-sdhost SD/MMC driver, optionally with overclock.
        N.B. This overlay is designed for situations where the mmc driver is
        the default, so it disables the other (mmc) interface - this will kill
        WiFi on a Pi3. If this isn't what you want, either use the sdtweak
        overlay or the new sd_* dtparams of the base DTBs.
Load:   dtoverlay=sdhost,<param>=<val>
Params: overclock_50            Clock (in MHz) to use when the MMC framework
                                requests 50MHz

        force_pio               Disable DMA support (default off)

        pio_limit               Number of blocks above which to use DMA
                                (default 1)

        debug                   Enable debug output (default off)


Name:   sdio
Info:   Selects the bcm2835-sdhost SD/MMC driver, optionally with overclock,
        and enables SDIO via GPIOs 22-27.
Load:   dtoverlay=sdio,<param>=<val>
Params: overclock_50            SD Clock (in MHz) to use when the MMC framework
                                requests 50MHz

        sdio_overclock          SDIO Clock (in MHz) to use when the MMC
                                framework requests 50MHz

        force_pio               Disable DMA support (default off)

        pio_limit               Number of blocks above which to use DMA
                                (default 1)

        debug                   Enable debug output (default off)

        poll_once               Disable SDIO-device polling every second
                                (default on: polling once at boot-time)

        bus_width               Set the SDIO host bus width (default 4 bits)


Name:   sdio-1bit
Info:   Selects the bcm2835-sdhost SD/MMC driver, optionally with overclock,
        and enables 1-bit SDIO via GPIOs 22-25.
Load:   dtoverlay=sdio-1bit,<param>=<val>
Params: overclock_50            SD Clock (in MHz) to use when the MMC framework
                                requests 50MHz

        sdio_overclock          SDIO Clock (in MHz) to use when the MMC
                                framework requests 50MHz

        force_pio               Disable DMA support (default off)

        pio_limit               Number of blocks above which to use DMA
                                (default 1)

        debug                   Enable debug output (default off)

        poll_once               Disable SDIO-device polling every second
                                (default on: polling once at boot-time)


Name:   sdtweak
Info:   Tunes the bcm2835-sdhost SD/MMC driver
        N.B. This functionality is now available via the sd_* dtparams in the
        base DTB.
Load:   dtoverlay=sdtweak,<param>=<val>
Params: overclock_50            Clock (in MHz) to use when the MMC framework
                                requests 50MHz

        force_pio               Disable DMA support (default off)

        pio_limit               Number of blocks above which to use DMA
                                (default 1)

        debug                   Enable debug output (default off)


Name:   smi
Info:   Enables the Secondary Memory Interface peripheral. Uses GPIOs 2-25!
Load:   dtoverlay=smi
Params: <None>


Name:   smi-dev
Info:   Enables the userspace interface for the SMI driver
Load:   dtoverlay=smi-dev
Params: <None>


Name:   smi-nand
Info:   Enables access to NAND flash via the SMI interface
Load:   dtoverlay=smi-nand
Params: <None>


Name:   spi-gpio35-39
Info:   Move SPI function block to GPIO 35 to 39
Load:   dtoverlay=spi-gpio35-39
Params: <None>


Name:   spi-rtc
Info:   Adds support for a number of SPI Real Time Clock devices
Load:   dtoverlay=spi-rtc,<param>=<val>
Params: pcf2123                 Select the PCF2123 device


Name:   spi0-cs
Info:   Allows the (software) CS pins for SPI0 to be changed
Load:   dtoverlay=spi0-cs,<param>=<val>
Params: cs0_pin                 GPIO pin for CS0 (default 8)
        cs1_pin                 GPIO pin for CS1 (default 7)


Name:   spi0-hw-cs
Info:   Re-enables hardware CS/CE (chip selects) for SPI0
Load:   dtoverlay=spi0-hw-cs
Params: <None>


Name:   spi1-1cs
Info:   Enables spi1 with a single chip select (CS) line and associated spidev
        dev node. The gpio pin number for the CS line and spidev device node
        creation are configurable.
        N.B.: spi1 is only accessible on devices with a 40pin header, eg:
              A+, B+, Zero and PI2 B; as well as the Compute Module.
Load:   dtoverlay=spi1-1cs,<param>=<val>
Params: cs0_pin                 GPIO pin for CS0 (default 18 - BCM SPI1_CE0).
        cs0_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev1.0 (default
                                is 'okay' or enabled).


Name:   spi1-2cs
Info:   Enables spi1 with two chip select (CS) lines and associated spidev
        dev nodes. The gpio pin numbers for the CS lines and spidev device node
        creation are configurable.
        N.B.: spi1 is only accessible on devices with a 40pin header, eg:
              A+, B+, Zero and PI2 B; as well as the Compute Module.
Load:   dtoverlay=spi1-2cs,<param>=<val>
Params: cs0_pin                 GPIO pin for CS0 (default 18 - BCM SPI1_CE0).
        cs1_pin                 GPIO pin for CS1 (default 17 - BCM SPI1_CE1).
        cs0_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev1.0 (default
                                is 'okay' or enabled).
        cs1_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev1.1 (default
                                is 'okay' or enabled).


Name:   spi1-3cs
Info:   Enables spi1 with three chip select (CS) lines and associated spidev
        dev nodes. The gpio pin numbers for the CS lines and spidev device node
        creation are configurable.
        N.B.: spi1 is only accessible on devices with a 40pin header, eg:
              A+, B+, Zero and PI2 B; as well as the Compute Module.
Load:   dtoverlay=spi1-3cs,<param>=<val>
Params: cs0_pin                 GPIO pin for CS0 (default 18 - BCM SPI1_CE0).
        cs1_pin                 GPIO pin for CS1 (default 17 - BCM SPI1_CE1).
        cs2_pin                 GPIO pin for CS2 (default 16 - BCM SPI1_CE2).
        cs0_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev1.0 (default
                                is 'okay' or enabled).
        cs1_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev1.1 (default
                                is 'okay' or enabled).
        cs2_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev1.2 (default
                                is 'okay' or enabled).


Name:   spi2-1cs
Info:   Enables spi2 with a single chip select (CS) line and associated spidev
        dev node. The gpio pin number for the CS line and spidev device node
        creation are configurable.
        N.B.: spi2 is only accessible with the Compute Module.
Load:   dtoverlay=spi2-1cs,<param>=<val>
Params: cs0_pin                 GPIO pin for CS0 (default 43 - BCM SPI2_CE0).
        cs0_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev2.0 (default
                                is 'okay' or enabled).


Name:   spi2-2cs
Info:   Enables spi2 with two chip select (CS) lines and associated spidev
        dev nodes. The gpio pin numbers for the CS lines and spidev device node
        creation are configurable.
        N.B.: spi2 is only accessible with the Compute Module.
Load:   dtoverlay=spi2-2cs,<param>=<val>
Params: cs0_pin                 GPIO pin for CS0 (default 43 - BCM SPI2_CE0).
        cs1_pin                 GPIO pin for CS1 (default 44 - BCM SPI2_CE1).
        cs0_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev2.0 (default
                                is 'okay' or enabled).
        cs1_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev2.1 (default
                                is 'okay' or enabled).


Name:   spi2-3cs
Info:   Enables spi2 with three chip select (CS) lines and associated spidev
        dev nodes. The gpio pin numbers for the CS lines and spidev device node
        creation are configurable.
        N.B.: spi2 is only accessible with the Compute Module.
Load:   dtoverlay=spi2-3cs,<param>=<val>
Params: cs0_pin                 GPIO pin for CS0 (default 43 - BCM SPI2_CE0).
        cs1_pin                 GPIO pin for CS1 (default 44 - BCM SPI2_CE1).
        cs2_pin                 GPIO pin for CS2 (default 45 - BCM SPI2_CE2).
        cs0_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev2.0 (default
                                is 'okay' or enabled).
        cs1_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev2.1 (default
                                is 'okay' or enabled).
        cs2_spidev              Set to 'disabled' to stop the creation of a
                                userspace device node /dev/spidev2.2 (default
                                is 'okay' or enabled).


Name:   tinylcd35
Info:   3.5" Color TFT Display by www.tinylcd.com
        Options: Touch, RTC, keypad
Load:   dtoverlay=tinylcd35,<param>=<val>
Params: speed                   Display SPI bus speed

        rotate                  Display rotation {0,90,180,270}

        fps                     Delay between frame updates

        debug                   Debug output level {0-7}

        touch                   Enable touch panel

        touchgpio               Touch controller IRQ GPIO

        xohms                   Touchpanel: Resistance of X-plate in ohms

        rtc-pcf                 PCF8563 Real Time Clock

        rtc-ds                  DS1307 Real Time Clock

        keypad                  Enable keypad

        Examples:
            Display with touchpanel, PCF8563 RTC and keypad:
                dtoverlay=tinylcd35,touch,rtc-pcf,keypad
            Old touch display:
                dtoverlay=tinylcd35,touch,touchgpio=3


Name:   uart1
Info:   Enable uart1 in place of uart0
Load:   dtoverlay=uart1,<param>=<val>
Params: txd1_pin                GPIO pin for TXD1 (14, 32 or 40 - default 14)

        rxd1_pin                GPIO pin for RXD1 (15, 33 or 41 - default 15)


Name:   vc4-fkms-v3d
Info:   Enable Eric Anholt's DRM VC4 V3D driver on top of the dispmanx
        display stack.
Load:   dtoverlay=vc4-fkms-v3d,<param>
Params: cma-256                 CMA is 256MB, 256MB-aligned (needs 1GB)
        cma-192                 CMA is 192MB, 256MB-aligned (needs 1GB)
        cma-128                 CMA is 128MB, 128MB-aligned
        cma-96                  CMA is 96MB, 128MB-aligned
        cma-64                  CMA is 64MB, 64MB-aligned


Name:   vc4-kms-v3d
Info:   Enable Eric Anholt's DRM VC4 HDMI/HVS/V3D driver. Running startx or
        booting to GUI while this overlay is in use will cause interesting
        lockups.
Load:   dtoverlay=vc4-kms-v3d,<param>
Params: cma-256                 CMA is 256MB, 256MB-aligned (needs 1GB)
        cma-192                 CMA is 192MB, 256MB-aligned (needs 1GB)
        cma-128                 CMA is 128MB, 128MB-aligned
        cma-96                  CMA is 96MB, 128MB-aligned
        cma-64                  CMA is 64MB, 64MB-aligned


Name:   vga666
Info:   Overlay for the Fen Logic VGA666 board
        This uses GPIOs 2-21 (so no I2C), and activates the output 2-3 seconds
        after the kernel has started.
Load:   dtoverlay=vga666
Params: <None>


Name:   w1-gpio
Info:   Configures the w1-gpio Onewire interface module.
        Use this overlay if you *don't* need a GPIO to drive an external pullup.
Load:   dtoverlay=w1-gpio,<param>=<val>
Params: gpiopin                 GPIO for I/O (default "4")

        pullup                  Non-zero, "on", or "y" to enable the parasitic
                                power (2-wire, power-on-data) feature


Name:   w1-gpio-pullup
Info:   Configures the w1-gpio Onewire interface module.
        Use this overlay if you *do* need a GPIO to drive an external pullup.
Load:   dtoverlay=w1-gpio-pullup,<param>=<val>
Params: gpiopin                 GPIO for I/O (default "4")

        pullup                  Non-zero, "on", or "y" to enable the parasitic
                                power (2-wire, power-on-data) feature

        extpullup               GPIO for external pullup (default "5")


Name:   wittypi
Info:   Configures the wittypi RTC module.
Load:   dtoverlay=wittypi,<param>=<val>
Params: led_gpio                GPIO for LED (default "17")
        led_trigger             Choose which activity the LED tracks (default
                                "default-on")


Troubleshooting
===============

If you are experiencing problems that you think are DT-related, enable DT
diagnostic output by adding this to /boot/config.txt:

    dtdebug=on

and rebooting. Then run:

    sudo vcdbg log msg

and look for relevant messages.

Further reading
===============

This is only meant to be a quick introduction to the subject of Device Tree on
Raspberry Pi. There is a more complete explanation here:

http://www.raspberrypi.org/documentation/configuration/device-tree.md
