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APU GPIO tutorial

PC Engines apu2 platform series have many features that are being controlled by GPIOs, in particular:

  • mPCIe resets
  • WLAN disable on mPCIe slots
  • SIM swaps
  • LEDs
  • S1 switch

In order to make it possible to modify/read their state, an OS driver is required. In Linux there is a pinctrl_amd module which is responsible for GPIO controller handling. The driver required special ACPI device definition for GPIO controller to work. Since version, the ACPI support was added and enabled GPIO interface via sysfc in Linux systems.


Linux has a special driver called gpio-leds which interacts with GPIO controller pinctrl driver to utilize a fancy sysfs interface for LED handling. All 3 front leds have been assigned to this driver and as a result are exposing following interface:

$ ls /sys/class/leds/
apu2:green:led1  apu2:green:led2  apu2:green:led3

The name of the platform in the leds filename changes according to platform. So apu3 will have like apu3:green:ledX

Each of these LEDs expose following interface:

$ ls /sys/class/leds/apu2\:green\:ledX
brightness  device  max_brightness  power  subsystem  trigger  uevent

The most interesting are brightness and trigger:

  1. brightness - as the name says, it can control the LED brightness.

GPIO controller only supports binary output (0 or 1) so the led can only be turned on or off

  • turn the led on: echo 1 > brightness
  • turn the led off: echo 0 > brightness
  • get current led state: cat brightness

  • trigger - is a string which defines the system activity that will cause the LED to be on or off. There are various triggers like: disk-activity, kbd-capslock, mmc0, heartbeat. Each of them can light the led up on certain event like disk activity, SD activity, keyboard special key etc.

  • to set a trigger: echo kbd-capslock > trigger

  • to unset a trigger: echo none > trigger

By default LED3 has been set as a heartbeat. heartbeat requires additional module to be loaded: ledtrig-heartbeat. In order to load the module by default add the ledtrig-heartbeat to /etc/modules-load.d/modules.conf.

S1 switch button

The small button near the SD card slot is called a S1 button. With the ACPI it has been configured to work with gpio-keys module which handles a interrupt GPIO keys/keyboards. S1 switch has following attributes:

  • active state low
  • edge triggered
  • pull-up by default
  • interrupt line 7
  • debounce time interval: 100ms

The driver is also set to use the S1 switch as EV_KEY event type and the key code to emit BTN_1 (257).

In order to determine whether the interrupt work for the S1 switch one can check the initial interrupts statistics with cat /proc/interrupts:

            CPU0       CPU1       CPU2       CPU3
   7:          0          0          0          1  IR-IO-APIC    7-fasteoi   pinctrl_amd
  58:          0          0          0          0  amd_gpio   89  switch1

After pressing the button few times:

            CPU0       CPU1       CPU2       CPU3
   7:          0          0          0         17  IR-IO-APIC    7-fasteoi   pinctrl_amd
  58:          0          0          0         16  amd_gpio   89  switch1

The button can be further used in user own applications.

Raw GPIO control

Other GPIO signals that do not have a dedicated river have to be controlled manually. In order to control a GPIO, a simple sysfs interface is introduced.

$ ls /sys/class/gpio/
export  gpiochip320  unexport

As one can see there is a gpiochip320 which corresponds to GPIO controller of the SoC. The 320 number according to documentation corresponds to the first GPIO number that can be controlled by this chip. The exact number of controllable GPIOs for this chip can be retrieved with:

$ cat /sys/class/gpio/gpiochip320/ngpio

As one can see this driver supports 192 GPIOs that can be controlled by this chip giving a range of 320-511 GPIO numbers. In order to control a GPIO one has to export the GPIO to sysfs first:

$ echo 391 > /sys/class/gpio/export

Note that only GPIOs from range 320-512 are supported. Writing other values will cause -bash: echo: write error: Invalid argument.

If the operation was successful a GPIO will appear:

$ ls /sys/class/gpio/
export  gpio391  gpiochip320  unexport

In fact we have exported a GPIO71 of the SoC, the 320 offset must be subtracted.

Each GPIO can export following interface:

$ ls /sys/class/gpio/gpio391
active_low  device  direction  edge  power  subsystem  uevent  value
  • active_low - indicates whether this pin is an active low signal (for input only)
  • direction - can be either in or out
  • edge - for inputs, whether pin should be active on rising, falling, both, none edge
  • value - state of the pin

  1. In order to change the state of the GPIO:

    # ensure pin is in output mode
    $ echo out > direction
    # check current state
    $ cat value
    # change the state
    $ echo 0 > value
    $ cat value

  2. Change pin direction:

    # set to input
    $ echo in > direction
    # set to output
    $ echo out > direction

  3. Set pin to active low:

    $ echo 1 > active_low

GPIO mappings

PC Engines apu series has many GPIOs and they differ between board versions. In order to not disrupt platform operation, only the following GPIOs should be exported:

  1. APU2:

  2. 386 - mPCIe1 reset

  3. 387 - mPCIe2 reset
  4. 391 - mPCIe1 WLAN disable
  5. 392 - mPCIe2 WLAN disable

  6. APU3/APU4:

  7. 386 - mPCIe3 reset

  8. 387 - mPCIe2 reset
  9. 391 - mPCIe3 WLAN disable
  10. 392 - mPCIe2 WLAN disable
  11. 410 - SIM swap

  12. APU5:

  13. 386 - mPCIe3 reset

  14. 387 - mPCIe2 reset
  15. 391 - mPCIe3 WLAN disable
  16. 392 - mPCIe2 WLAN disable
  17. 410 - SIM swap

Important: when exporting, pin changes its state to input and low state. In BIOS these pins are configured as output high. Be sure to configure them to output first before using them.

Known issues

  1. Since coreboot release v4.10.0.0 there is conflict with apuled driver in FreeBSD. It is because BIOS reserves resources for GPIOs with ACPI controller support. As a result, FreeBSD cannot reserve the memory for native driver anymore. Entire issue with details is described here.

    The workaround was found by alexpro user and is simply adding environment variable debug.acpi.avoid="\_SB_.PCI0.GPIO" to loader.conf. It doesn't affect any other ACPI functionality then GPIOs.

  2. LED entries in sysfs in Linux are duplicated due to presence of leds_apu module. Blacklisting leds_apu module will get rid of the duplicates. See issue