On Wed, 02 Mar, at 09:29:17PM, Chen Yu wrote:

The problem is Linux registers pm_power_off = efi_power_off
only if we are in hardware reduced mode. Actually, what we also
want is to do this when ACPI S5 is simply not supported.
That should handle both the HW reduced mode, and the HW-full
mode where the DSDT fails to supply an _S5 object.

This patch fixes this issue by introducing a new flag acpi_no_s5 which
indicates the non-existence of _S5. The initial state of acpi_no_s5 is
false and probed in acpi_sleep_init, then we'll later see the updated
value in efi_poweroff_required, according to which we can set pm_power_off
to efi_power_off in efi_shutdown_init.

Suggested-by: Len Brown <redacted>
Signed-off-by: Chen Yu <redacted>
---
 arch/x86/platform/efi/quirks.c | 3 ++-
 drivers/acpi/sleep.c           | 2 ++
 include/acpi/acpixf.h          | 6 ++++++
 3 files changed, 10 insertions(+), 1 deletion(-)

Are there legacy platforms without _S5 where we currently handle
reboot via some other means that are going to break if we attempt an
EFI reboot?

I ask because historically performing an EFI reboot has been all kinds
of buggy on x86.

Also, is this what Windows does if _S5 is absent?