GPIO in the kernel: an introduction

GPIO in the kernel: an introduction

Also those quality specialists tell you to use the macro:
#define IOWRITE32BYTE(addr, val) *((volatile unsigned *)(addr)) = (val)
(Hint: try to write the same addr twice with a newer compiler).

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;