Re: EFI reboot vs. ACPI reboot (was: Re: [tip:x86/urgent] x86/reboot, efi: Use EFI reboot for Acer TravelMate X514-51T)

[Daniel Drake]

On Wed, Apr 17, 2019 at 2:16 PM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> Ok, so acpi_gbl_reduced_hardware is set when the ... 'reduced hardware'
> bit is set:
>
>         acpi_gbl_reduced_hardware = FALSE;
>         if (acpi_gbl_FADT.flags & ACPI_FADT_HW_REDUCED) {
>                 acpi_gbl_reduced_hardware = TRUE;
>         }
>
>
> which is described as:
>
>  #define ACPI_FADT_HW_REDUCED        (1<<20)     /* 20: [V5] ACPI hardware is not implemented (ACPI 5.0) */
>
> That seems counter-intuitive to me: if no full ACPI hardware is
> implemented then we should assume reduced ACPI functionality, i.e. if the
> EFI runtime is otherwise available we should default to it.

It's a bit confusing, but my loose understanding is that previous
versions of the ACPI spec required system implementors to implement
the whole thing; but that's increasingly impractical today, e.g. with
ARM systems coming along, which do not gel well with some of the
historical x86-rooted design aspects that spilled over into ACPI. The
V5 spec introduces reduced mode as an opt-in new feature, but for
compatibility with pre-V5 implementations it needs to consider "full
hardware" mode as the default.

> Feel free to send a patch that makes EFI reboot the default one under
> these circumstances,

Just to check, you mean: EFI reboot (and shutdown) become the default
methods when the machine is booted in EFI mode, and EFI stuff has not
been disabled with a kernel parameter?
Even when running in full hardware ACPI mode.

Thanks
Daniel