Re: [PATCH] rtc: Disable EFI rtc for x86

H. Peter Anvin
2014-11-10T20:59:20Z

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