grub2 中时间的获取在arm和arm64的差别

来源：互联网发布：鸟哥的linux私房菜简体编辑：程序博客网时间：2024/06/05 11:27

在arm32的时候，在grub中通过调用bios提供的create_event 来产生时间戳，源码路径如下：grub-core/kernel/arm/efi/init.c
increment_timer (grub_efi_event_t event __attribute__ ((unused)),
       void *context __attribute__ ((unused)))
{
tmr += 10;
}

void
grub_machine_init (void)
{
grub_efi_boot_services_t *b;

grub_efi_init ();

b = grub_efi_system_table->boot_services;

efi_call_5 (b->create_event, GRUB_EFI_EVT_TIMER | GRUB_EFI_EVT_NOTIFY_SIGNAL,
          GRUB_EFI_TPL_CALLBACK, increment_timer, NULL, &tmr_evt);
efi_call_3 (b->set_timer, tmr_evt, GRUB_EFI_TIMER_PERIODIC, 100000);

grub_install_get_time_ms (grub_efi_get_time_ms);
}

grub_uint64_t
grub_get_time_ms (void)
{
return get_time_ms_func ();
}

void
grub_install_get_time_ms (get_time_ms_func_t func)
{
get_time_ms_func = func;
}

后续在driver中只要调用grub_get_time_ms 就可以获得时间，例如下面这段code
if (! (hba->bios_handoff & GRUB_AHCI_BIOS_HANDOFF_OS_OWNED))
    {
      grub_uint64_t endtime;

      grub_dprintf ("ahci", "Requesting AHCI ownership\n");
      hba->bios_handoff = (hba->bios_handoff & ~GRUB_AHCI_BIOS_HANDOFF_RWC)
   | GRUB_AHCI_BIOS_HANDOFF_OS_OWNED;
      grub_dprintf ("ahci", "Waiting for BIOS to give up ownership\n");
      endtime = grub_get_time_ms () + 1000;
}

而在arm64的时候就直接可以通过寄存器获得时间戳了，源码路径grub-core/kernel/arm64/efi/init.c
static grub_uint64_t
grub_efi_get_time_ms (void)
{
grub_uint64_t tmr;
asm volatile("mrs %0,   cntvct_el0" : "=r" (tmr));

return tmr / timer_frequency_in_khz;
}

void
grub_machine_init (void)
{
grub_uint64_t timer_frequency;

grub_efi_init ();

asm volatile("mrs %0,   cntfrq_el0" : "=r" (timer_frequency));
timer_frequency_in_khz = timer_frequency / 1000;

grub_install_get_time_ms (grub_efi_get_time_ms);
}

可见只要读取cntvct_el0 这个寄存器就可以获得时间

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