smol-gilbraltar/kernel/mbox.c

#include <assert.h>
#include <mbox.h>
#include <mem.h>

uint32_t gilbraltar_mbox_write_read(uintptr_t dma) {
  assert((dma & 3) == 0); // [dma] must be aligned to 32-bit

  dma |= 0xc0000000U;

  while (read32(MAILBOX1_STATUS) & MAILBOX_STATUS_FULL)
    ;
  write32(MAILBOX1_WRITE, BCM_MAILBOX_PROP_OUT | dma);

  uint32_t result;

  do {
    while (read32(MAILBOX0_STATUS) & MAILBOX_STATUS_EMPTY)
      ;
    result = read32(MAILBOX0_READ);
  } while ((result & 0xf) != BCM_MAILBOX_PROP_OUT);

  return ((result & ~0xf) & 0x3fffffff);
}