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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Date:   Wed, 17 Apr 2019 09:51:47 -0700
From:   hpa@...or.com
To:     Ingo Molnar <mingo@...nel.org>
CC:     Daniel Drake <drake@...lessm.com>,
        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 9:37:29 AM PDT, Ingo Molnar <mingo@...nel.org> wrote:
>
>* hpa@...or.com <hpa@...or.com> wrote:
>
>> > 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.
>
>No, I still think "early" EFI is historically better with ACPI reboot.
>
>But can we find a firmware flag perhaps that will *not* result in EFI 
>reboot being turned off?
>
>> This, I believe, is known to not work.
>
>Yeah, I bet so.
>
>My problem is that the code appears to have the wrong assumptions:
>
> /*
>* For most modern platforms the preferred method of powering off is via
>  * ACPI. However, there are some that are known to require the use of
>  * EFI runtime services and for which ACPI does not work at all.
>  *
>  * Using EFI is a last resort, to be used only if no other option
>  * exists.
>  */
> bool efi_reboot_required(void)
> {
>         if (!acpi_gbl_reduced_hardware)
>                 return false;
>
>         efi_reboot_quirk_mode = EFI_RESET_WARM;
>         return true;
> }
>
>
>At minimum the comment is stale: "modern" platforms, *especially* when 
>the only bootup method is EFI, as in the ACER laptop case, I think the 
>preferred reboot method is absolutely an EFI reboot - and it's probably
>
>what Windows uses too.
>
>The question is, is acpi_gbl_reduced_hardware false on the Acer 
>TravelMate X514-51T? I think it has to be, for the quirk to make sense
>- 
>if it's true then efi_reboot_required() would set the reboot method to 
>EFI.
>
>I.e. we seem to have a new category of systems that are advertising 
>themselves as 'full ACPI compliant', which are NOT old EFI systems, but
>
>modern EFI systems.
>
>Is there some good way to detect these - such as ACPI version or 
>something?
>
>Thanks,
>
>	Ingo

That is exactly what the reduced hardware flag is supposed to indicate. As far as what Windows does, the are only two ways to find out: testing somehow, or these days Microsoft might actually answer if we ask nicely.
-- 
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