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Date: Mon, 10 Nov 2014 12:58:39 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Austin S Hemmelgarn <ahferroin7@gmail.com>,
        Matthew Garrett <mjg59@srcf.ucam.org>
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        linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Dave Young <dyoung@redhat.com>
Subject: Re: [PATCH] rtc: Disable EFI rtc for x86
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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