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

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Date:	Fri, 14 Nov 2014 12:21:02 +0100
From:	Pali Rohár <pali.rohar@...il.com>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>
Cc:	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>,
	"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 17:45:06 Austin S Hemmelgarn wrote:
> 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.
> 

So how to solve it? I think that something like efi=rtc_enable 
param is better then disabling it for x86 at compile time.

Once we figure out which machines are affected or if in future 
vendors fix their firmwares it can be enabled for new machines...

-- 
Pali Rohár
pali.rohar@...il.com

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