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 18:37:29 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     hpa@...or.com
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)

* 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