GPIO in the kernel: an introduction

GPIO lines seem to be especially prevalent in embedded systems; even so, there never seems to be enough of them. As one might expect, a system with dozens (or even hundreds) of GPIOs needs some sort of rational abstraction for managing them. The kernel has had such a mechanism since 2.6.21 (it was initially added by David Brownell). The API has changed surprisingly little since then, but that period of relative stasis may be about to come about to an end. The intended changes are best understood in the context of the existing API, though, so that is what this article will cover. Subsequent installments will look at how the GPIO API may evolve in the near future.

Naturally, there is an include file for working with GPIOs:

    #include <linux/gpio.h>

In current kernels, every GPIO in the system is represented by a simple unsigned integer. There is no provision for somehow mapping a desired function ("the sensor power line for the first camera device," say) onto a GPIO number; the code must come by that knowledge by other means. Often that is done through a long series of macro definitions; it is also possible to pass GPIO numbers through platform data or a device tree.

GPIOs must be allocated before use, though the current implementation does not enforce this requirement. The basic allocation function is:

    int gpio_request(unsigned int gpio, const char *label);

The gpio parameter indicates which GPIO is required, while label associates a string with it that can later appear in sysfs. The usual convention applies: a zero return code indicates success; otherwise the return value will be a negative error number. A GPIO can be returned to the system with:

    void gpio_free(unsigned int gpio);

There are some variants of these functions; gpio_request_one() can be used to set the initial configuration of the GPIO, and gpio_request_array() can request and configure a whole set of GPIOs with a single call. There are also "managed" versions (devm_gpio_request(), for example) that automatically handle cleanup if the developer forgets.

Some GPIOs are used for output, others for input. A suitably-wired GPIO can be used in either mode, though only one direction is active at any given time. Kernel code must inform the GPIO core of how a line is to be used; that is done with these functions:

    int gpio_direction_input(unsigned int gpio);
    int gpio_direction_output(unsigned int gpio, int value);

In either case, gpio is the GPIO number. In the output case, the value of the GPIO (zero or one) must also be specified; the GPIO will be set accordingly as part of the call. For both functions, the return value is again zero or a negative error number. The direction of (suitably capable) GPIOs can be changed at any time.

For input GPIOs, the current value can be read with:

    int gpio_get_value(unsigned int gpio);

This function returns the value of the provided gpio; it has no provision for returning an error code. It is assumed (correctly in almost all cases) that any errors will be found when gpio_direction_input() is called, so checking the return value from that function is important.

Setting the value of output GPIOs can always be done using gpio_direction_output(), but, if the GPIO is known to be in output mode already, gpio_set_value() may be a bit more efficient:

    void gpio_set_value(unsigned int gpio, int value);

Some GPIO controllers can generate interrupts when an input GPIO changes value. In such cases, code wishing to handle such interrupts should start by determining which IRQ number is associated with a given GPIO line:

    int gpio_to_irq(unsigned int gpio);

The given gpio must have been obtained with gpio_request() and put into the input mode first. If there is an associated interrupt number, it will be passed back as the return value from gpio_to_irq(); otherwise a negative error number will be returned. Once obtained in this manner, the interrupt number can be passed to request_irq() to set up the handling of the interrupt.

Finally, the GPIO subsystem is able to represent GPIO lines via a sysfs hierarchy, allowing user space to query (and possibly modify) them. Kernel code can cause a specific GPIO to appear in sysfs with:

    int gpio_export(unsigned int gpio, bool direction_may_change);

The direction_may_change parameter controls whether user space is allowed to change the direction of the GPIO; in many cases, allowing that control would be asking for bad things to happen to the system as a whole. A GPIO can be removed from sysfs with gpio_unexport() or given another name with gpio_export_link().

And that is an overview of the kernel's low-level GPIO interface. A number of details have naturally been left out; see Documentation/gpio.txt for a more thorough description. Also omitted is the low-level driver's side of the API, by which GPIO lines can be made available to the GPIO subsystem; covering that API may be the subject of a future article. The next installment, though, will look at a couple of perceived deficiencies in the above-described API and how they might be remedied.

The volatile should ensure that both writes occur in that case.

Anyway on newer architecture, those are memory mapped and IHMO it is a lot cleaner to use C to declare them:
volatile struct my_IO_s {
enum { healthy, fail } power_state : 1;
unsigned power_active : 1;
} * const my_IO = (volatile struct my_IO_s *)0xDC002000;
Instead of pages and pages of #define.

I agree, and I do the same in embedded development, but it's not perfect. The three biggest problems:

• You lose control over the access size. It can vary between compilers, compiler versions, architectures, and even ABIs for particular architectures. For example, see GCC bug #23623
• They don't map well to unusual registers. Consider an interrupt flag register which behaves like:

      read 0: interrupt has not occurred
      read 1: interrupt has occurred
      write 0: no action
      write 1: clear interrupt flag
  

  To clear a flag, you can't just do:

      my_IO->timer_interrupt = 1;
  

  because this can become

      tmp = *my_IO_addr | (1 << TIMER_INTERRUPT_SHIFT);
      *my_IO_addr = tmp;
  

  Instead, you'd need to use something like:

      *my_IO = (struct my_IO_s) {
          .timer_interrupt = 1
      };
  

  which is not much better than the equivalent:

      *my_IO_addr = (1 << TIMER_INTERRUPT_SHIFT);
  

• Even for a more normal register like a GPIO port, it's quite difficult to set two bits simultaneously with a single write. You can't do:

      *my_IO_addr |= (1 << PIN0_SHIFT) | (1 << PIN1_SHIFT);
  

  without something like:

     struct my_IO_s tmp = *my_IO;
     tmp.pin0 = 1;
     tmp.pin1 = 1;
     *my_IO = tmp;
  

1. GPIO can be both output and input at the same time. Yep you may want to read back the value of an output.

Indeed this is an absolute requirement for a GPIO used as an I2C SCL line.