Linux PSP GPIO Driver Guide

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Linux PSP GPIO Driver Guide
Linux PSP

Introduction

This wiki page provides the usage information of GPIO Linux driver usage, both in user and kernel space.

Set pinmux configuration for GPIO pin usage, details of pinmux configuration are mentioned on PSP user guide. If the pinmux settings are not correct then the GPIO will not function as expected.

NOTE

Linux GPIO numbers start from "0".


NOTE

Some TI SoCs have multiple banks of GPIOs.

For example the AM335x has 4 banks of 32 GPIOs, for a total of 128 GPIO, i.e Bank-0[0..31], Bank-1[0..31], Bank-2[0..31], Bank-3[0..31].

In this case Bank-1[0..31] maps to Linux GPIO numbers 32..63, Bank-2[0..31] to Linux GPIO numbers 64..95 etc.


Driver Configuration

This section describes about the kernel configurations for GPIO driver

The default kernel configuration enables support for GPIO driver (built into the kernel).

To enable or disable GPIO driver from kernel build, follow these steps:

$ make CROSS_COMPILE=arm-none-linux-gnueabi- ARCH=arm menuconfig
  • Select Device Drivers from the main menu.
    Power management options  --->
[*] Networking support  --->
    Device Drivers  --->
    File systems  --->
...
...
  • Select GPIO Support from the menu.
    PPS support  --->
    PTP clock support  --->
-*- GPIO Support  --->
< > Dallas's 1-wire support  --->
...
...


Sysfs entries configuration

GPIO can be access using SYSFS entries from User Space. For that Select /sys/class/gpio/... (sysfs interface) from the GPIO support.

[ ] Debug GPIO calls
[*] /sys/class/gpio/... (sysfs interface)
    *** Memory mapped GPIO drivers: ***
< > Basic memory-mapped GPIO controllers support
...
...
  • After doing driver selection, exit and save the kernel configuration when prompted.


IRQ handling

GPIO pin is also used as a interrupt source, these are the general usage of IRQ handling using GPIO lines.

  • Map GPIO number to corresponding IRQ number, GPIO 0 need not use IRQ 0
irq_num = gpio_to_irq(30)
  • Request IRQ, make sure that irq_num should be non-error value
request_irq(irq_num, handler, 0, "gpio_test", NULL);
  • Set IRQ type Raising/Falling/Level triggered
set_irq_type(irq_num, IRQ_TYPE_EDGE_RISING);
  • During the clean-up path free the IRQ and gpio
free_irq(irq_num, NULL);
gpio_free(30);


Driver Usage

Kernel Level

  • Allocate memory to GPIO line, can be achieved by doing gpio_request()
err = gpio_request(30, "sample_name");

  • Depending on the requirement set GPIO as input or output pin then set gpio value as high or low.

Setting the GPIO pin 30 as input

gpio_direction_input(30);

Make pin 30 as output and set the value as high.

gpio_direction_output(30, 1);

Exporting that particular pin (30) to sysfs entry then use this API

gpio_export(30, true);

Get value from GPIO pin

gpio_get_value(30);


User Space - Sysfs control

  • Enable GPIO sysfs support in kernel configuration and build the kernel
Device Drivers  ---> GPIO Support  ---> /sys/class/gpio/... (sysfs interface)
  • Sysfs entries
Export the particular GPIO pin for user control. GPIO30 is taken as example.
$ echo 30 > /sys/class/gpio/export
Change the GPIO pin direction to in/out
$ echo "out" > /sys/class/gpio/gpio30/direction

or

$ echo "in" > /sys/class/gpio/gpio30/direction
Change the value
$ echo 1 > /sys/class/gpio/gpio30/value

or

$ echo 0 > /sys/class/gpio/gpio30/value
Unexport the GPIO pin
$ echo 30 > /sys/class/gpio/unexport

Note: GPIO's which are used already in the drivers can not be control from sysfs, unless until driver export that particular pin.

Run these commands for knowing what are the GPIO's already requested in the drivers.

$ mount -t debugfs debugfs /sys/kernel/debug
$ cat /sys/kernel/debug/gpio


References