linux-kernel - Re: [PATCH 1/3] rtc-efi: register rtc-efi device when EFI enabled

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Date:	Fri, 28 Dec 2012 15:44:45 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Matthew Garrett <matthew.garrett@...ula.com>
CC:	"Lee, Chun-Yi" <joeyli.kernel@...il.com>,
	"matt.fleming@...el.com" <matt.fleming@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"Lee, Chun-Yi" <jlee@...e.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Jan Beulich <JBeulich@...e.com>, Len Brown <lenb@...nel.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH 1/3] rtc-efi: register rtc-efi device when EFI enabled

On 12/28/2012 12:49 PM, Matthew Garrett wrote:
> On Fri, 2012-12-28 at 12:40 -0800, H. Peter Anvin wrote:
> 
>>> I suspect that what we *should* do looks like:
>>>
>>> 1. If ACPI exports a Time and Alarm Device (ACPI000E) the use it;
>>> 2. If ACPI exports an PC/AT device (PNP0B00/1/2) then use it(*);
>>> 3. If we have an EFI RTC use it;
>>> 4. Probe for a PC/AT RTC device.
> 
> In terms of ordering, 3 should probably come before 2 - but that depends
> on us actually fixing the issues that are preventing some of these calls
> from working. As far as wallclock time goes, EFI is going to be
> available to us before we've parsed the DSDT to determine whether
> there's any ACPI devices, so we'll almost certainly end up having to use
> it at at least some point during boot. Otherwise, agreed.
> 

[Resending due to misconfigured tablet]

3 before 2 is exactly what is known to break on existing hardware (ASUS).

If anything, we should move to using the EFI RTC as a very last resort,
i.e. 3 after 4.  Ideally we should match Win8 behavior, but that would
require someone mocking up different ACPI and EFI functions in a
simulator and see how Win8 prioritizes things.

	-hpa

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