linux-kernel - Re: [PATCH] rtc: Disable EFI rtc for x86

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Date:	Mon, 10 Nov 2014 12:58:39 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Matthew Garrett <mjg59@...f.ucam.org>
CC:	Pali Rohár <pali.rohar@...il.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Borislav Petkov <bp@...en8.de>,
	Mark Salter <msalter@...hat.com>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	rtc-linux@...glegroups.com, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org, Dave Young <dyoung@...hat.com>
Subject: Re: [PATCH] rtc: Disable EFI rtc for x86

On 11/10/2014 12:37 PM, Austin S Hemmelgarn wrote:
> On 2014-11-10 12:04, Matthew Garrett wrote:
>> On Mon, Nov 10, 2014 at 11:45:06AM -0500, Austin S Hemmelgarn wrote:
>>
>>> I agree, without it you need PC CMOS RTC support to access the RTC
>>> on most systems, which in turn means that you have to enable the CSM
>>> in the EFI firmware, which is annoying cause you can't easily dual
>>> boot windows with secure boot when the CSM is enabled.
>>
>> CMOS RTC support doesn't depend on the CSM.
>>
> That's really interesting, because with it compiled in, I can't boot on
> my EFI based thinkpad laptop without telling EFI to launch the CSM, and
> with it compiled out, I can boot fine without the CSM.  I'll have to
> look further into the options I have set in my kernel build, I may have
> changed something else without remembering between booting with and
> without the CSM enabled.
> 

It could also be that the non-CSM BIOS somehow remaps the CMOS registers.

	-hpa

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