EBC Exercise 51 GPIO aggregator

Embedded Linux Class by Mark A. Yoder

Here are some quick notes on the GPIO aggregator which allows one to group a number of GPIOs into a virtual GPIO chip, visible as an additional /dev/gpiochip*. Its documentation can be found in Documentation/admin-guide/gpio/gpio-aggregator.rst.

My info comes from:

https://bootlin.com/blog/gpio-aggregator-a-virtual-gpio-chip/

https://elixir.bootlin.com/linux/v5.12/source/Documentation/admin-guide/gpio/gpio-aggregator.rst

https://elinux.org/images/c/cb/Linux_GPIO-Evolution_and_Current_State_of_the_User_API.pdf

Install

bone$ grep -i gpio_agg /boot/config-5.10.120-ti-r48 
CONFIG_GPIO_AGGREGATOR=m
bone$ sudo modprobe gpio-aggregator
bone$ cd /sys/bus/platform/drivers/gpio-aggregator
bone$ ls -ls
total 0
0 --w------- 1 root root 4096 Jul 29 10:57 bind
0 --w------- 1 root root 4096 Jul 29 10:57 delete_device
0 lrwxrwxrwx 1 root root    0 Jul 29 10:57 module -> ../../../../module/gpio_aggregator
0 --w------- 1 root root 4096 Jul 29 10:57 new_device
0 --w------- 1 root root 4096 Jul 29 10:56 uevent
0 --w------- 1 root root 4096 Jul 29 10:57 unbind
bone$ sudo chgrp gpio *
bone$ sudo chmod g+w *
bone$ ls -ls
total 0
0 --w--w---- 1 root gpio 4096 Jul 29 11:09 bind
0 --w--w---- 1 root gpio 4096 Jul 29 11:09 delete_device
0 lrwxrwxrwx 1 root root    0 Jul 29 10:57 module -> ../../../../module/gpio_aggregator
0 --w--w---- 1 root gpio 4096 Jul 29 11:09 new_device
0 --w--w---- 1 root gpio 4096 Jul 29 11:09 uevent
0 --w--w---- 1 root gpio 4096 Jul 29 11:09 unbind
bone$ gpiodetect 
gpiochip0 [gpio-0-31] (32 lines)
gpiochip1 [gpio-32-63] (32 lines)
gpiochip2 [gpio-64-95] (32 lines)
gpiochip3 [gpio-96-127] (32 lines)

Map pins

Let's map P9_14, P9_16, P9_18 and P9_22 to lines 0-3 on a new gpiochip.

bone$ gpioinfo | grep -e chip -e P9_1[468] -e P9_22
gpiochip0 - 32 lines: 
	line   2: "P9_22 [spi0_sclk]" "gpio-aggregator.0" input active-high [used]
	line   4: "P9_18 [spi0_d1]" "gpio-aggregator.0" output active-high [used]
gpiochip1 - 32 lines:
	line  18: "P9_14 [ehrpwm1a]" "gpio-aggregator.0" output active-high [used]
	line  19: "P9_16 [ehrpwm1b]" "gpio-aggregator.0" input active-high [used]
gpiochip2 - 32 lines:
gpiochip3 - 32 lines:

Header	gpiochip	line
P9_14	1	18
P9_16	1	19
P9_18	0	4
P9_22	0	2

bone$ echo "gpio-32-63 18,19 gpio-0-31 4,2" > new_device
bone$ ls
bind  delete_device  gpio-aggregator.0  module  new_device  uevent  unbind
bone$ ls gpio-aggregator.0
driver  driver_override  gpio  gpiochip4  modalias  power  subsystem  uevent
bone$ gpioinfo | tail -6
	line  31:         "NC"       unused   input  active-high 
gpiochip4 - 4 lines:
	line   0:      unnamed       unused  output  active-high 
	line   1:      unnamed       unused   input  active-high 
	line   2:      unnamed       unused   input  active-high 
	line   3:      unnamed       unused  output  active-high 

bone$ gpiodetect 
gpiochip0 [gpio-0-31] (32 lines)
gpiochip1 [gpio-32-63] (32 lines)
gpiochip2 [gpio-64-95] (32 lines)
gpiochip3 [gpio-96-127] (32 lines)
gpiochip4 [gpio-aggregator.0] (4 lines)

There's a new gpiochip, gpiochip4

Test it

The new mappings are:

Header	gpiochip	line
P9_14	4	0
P9_16	4	1
P9_18	4	2
P9_22	4	3

Turn them all on, then off. Since they are all now on the same chip, one command can be used.

bone$ gpioset gpiochip4 0=1 1=1 2=1 3=1
bone$ gpioset gpiochip4 0=0 1=0 2=0 3=0

It's interesting the LEDs stay on, even after the command exits. Normally gpioset returns to it's original state.

Delete it

bone$ echo gpio-aggregator.0 > delete_device 

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Embedded Linux Class by Mark A. Yoder