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Date:	Mon, 10 Nov 2014 11:45:06 -0500
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Pali Rohár <pali.rohar@...il.com>,
	Matt Fleming <matt.fleming@...el.com>
CC:	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>,
	"H. Peter Anvin" <hpa@...or.com>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [PATCH] rtc: Disable EFI rtc for x86

On 2014-11-10 11:23, Pali Rohár wrote:
> On Monday 10 November 2014 12:22:13 Matt Fleming wrote:
>> On Sun, 2014-11-09 at 19:22 +0100, Borislav Petkov wrote:
>>> On Sun, Nov 09, 2014 at 06:37:46PM +0100, Pali Rohár wrote:
>>>> this patch totally disabled efi rfc driver on x86 machines
>>>> at compile time. But on some x86 machines it working
>>>> without crash and reading from file
>>>> /sys/class/rtc/rtc*/since_epoch returns correct
>>>> information. So why to disable compiling driver on
>>>> machines where driver working?
>>>
>>> Sounds like we need an efi=rtc_enable knob for people who
>>> what to use it...
>>
>> I'm not so sure. By and large, the EFI runtime Time services
>> just don't work very well on x86. Just because they work
>> sometimes, doesn't actually mean it's a good idea to use
>> them.
>>
>> The one scenario where the time services are useful is early
>> on during boot when we want to get the timezone information.
>> I have vague recollections of someone working on that.
>
> On laptop Dell Latitude E6440 with 64bit kernel 3.17-rc6 (where
> was rtc-efi.ko enabled at compile time for x86) it working fine
> without crash. So I think that other Dell machines could work
> too.
>
I can also confirm that it works correctly on a number of ASUS and 
ASrock desktop motherboards with the most recent BIOS updates, and on a 
Lenovo Thinkpad L540.  I can however confirm that it DOES NOT work 
however on an Acer Aspire V5-131 system, though I had never updated the 
BIOS on that one, so it may work correctly with a newer BIOS version.
> Maybe problems which were reported are only specific for 32bit
> UEFI? Or 32bit kernels?
>
> And I think that if I know that efi time service working on
> machine, I should be able to use it with mainline kernel (without
> hacking Kconfig to enable it and so...).
>
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.


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