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

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Date:	Mon, 10 Nov 2014 17:23:46 +0100
From:	Pali Rohár <pali.rohar@...il.com>
To:	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 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.

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...).

-- 
Pali Rohár
pali.rohar@...il.com

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