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

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Message-ID: <17516564-37AC-4F77-9C0D-0DD71F8045EA@zytor.com>
Date:   Wed, 17 Apr 2019 08:15:55 -0700
From:   hpa@...or.com
To:     Daniel Drake <drake@...lessm.com>, Ingo Molnar <mingo@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jian-Hong Pan <jian-hong@...lessm.com>,
        matt@...eblueprint.co.uk,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-efi@...r.kernel.org, Len Brown <len.brown@...el.com>,
        linux-tip-commits@...r.kernel.org,
        Endless Linux Upstreaming Team <linux@...lessm.com>
Subject: Re: EFI reboot vs. ACPI reboot (was: Re: [tip:x86/urgent] x86/reboot, efi: Use EFI reboot for Acer TravelMate X514-51T)

On April 17, 2019 5:38:41 AM PDT, Daniel Drake <drake@...lessm.com> wrote:
>On Wed, Apr 17, 2019 at 2:16 PM Ingo Molnar <mingo@...nel.org> 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

This, I believe, is known to not work.
-- 
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