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

From: Matt Fleming
Date: Wed Jan 02 2013 - 02:26:28 EST

On Wed, 2013-01-02 at 10:45 +0800, joeyli wrote:
> I didn't find the EFI RTC problem on Asus from google.
> Could you please share what's the situation on ASUS hardware?

It's mentioned in the following commit (which has since been reverted in
Linus' tree),

commit 185034e72d591f9465e5e18f937ed642e7ea0070
Author: Matt Fleming <matt.fleming@xxxxxxxxx>
Date: Fri Sep 7 18:28:04 2012 +0100

x86, efi: 1:1 pagetable mapping for virtual EFI calls

Some firmware still needs a 1:1 (virt->phys) mapping even after we've
called SetVirtualAddressMap(). So install the mapping alongside our
existing kernel mapping whenever we make EFI calls in virtual mode.

This bug was discovered on ASUS machines where the firmware
implementation of GetTime() accesses the RTC device via physical
addresses, even though that's bogus per the UEFI spec since we've
informed the firmware via SetVirtualAddressMap() that the boottime
memory map is no longer valid.

This bug seems to be present in a lot of consumer devices, so there's
not a lot we can do about this spec violation apart from workaround

The bug was originally mentioned here,

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