GPIO Overview

GPIO Driver
Notes from gpio in kernel docs

GPIO Driver

Introduction

site: http://processors.wiki.ti.com/index.php/TI81XX_PSP_GPIO_Driver_User_Guide

NOTE:

Linux GPIO numbers start from "0".

NOTE

Some TI SoCs have multiple banks of GPIOs. For example the TI814x 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

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
```

Notes from gpio in kernel docs

http://www.kernel.org/doc/Documentation/gpio.txt