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

["H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>]

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