linux-kernel - Re: [PATCH] clocksource: arch_timer: Fix code to use physical timers when requested

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Date:	Thu, 28 Aug 2014 17:10:49 -0700
From:	Sonny Rao <sonnyrao@...omium.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Doug Anderson <dianders@...omium.org>,
	Olof Johansson <olof@...om.net>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Russell King <linux@....linux.org.uk>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clocksource: arch_timer: Fix code to use physical timers
 when requested

On Thu, Aug 28, 2014 at 2:35 AM, Mark Rutland <mark.rutland@....com> wrote:
> On Thu, Aug 28, 2014 at 04:33:31AM +0100, Doug Anderson wrote:
>> Hi,
>>
>> On Wed, Aug 27, 2014 at 7:58 PM, Olof Johansson <olof@...om.net> wrote:
>> > On Wed, Aug 27, 2014 at 5:56 PM, Stephen Boyd <sboyd@...eaurora.org> wrote:
>> >> On 08/27/14 15:33, Olof Johansson wrote:
>> >>> On Wed, Aug 27, 2014 at 3:26 PM, Stephen Boyd <sboyd@...eaurora.org> wrote:
>> >>>
>> >>>> Is there any reason why the virtual counter can't be read? Maybe we're
>> >>>> the hyp and we need to make sure we don't use the virtual timer so that
>> >>>> the guest can use it, but that doesn't have any effect on the usage of
>> >>>> the virtual counter for the clocksource.
>> >>> There are several cases where virtual is unusable -- in particular it
>> >>> might not have been configured properly (i.e. the phys/virt offset is
>> >>> at a bad value).
>> >>>
>> >>
>> >> Any specifics? It would be nice to say so in the commit text so that
>> >> others using such devices know they need this patch. I'm guessing the
>> >> firmware can't be fixed?
>>
>> Even if we could change things to use a virtual timer in some cases,
>> Sonny's patch still fixes a bug.  The code as written right now makes
>> pretenses about supporting the physical timer, but it doesn't work.
>> That should be fixed.
>
> The code does support the physical timer. It does not support the
> physical counter (and makes no pretenses that it does).
>

Is there some reason that it should not support it?  It seems like the
two things are highly related.

> I had hoped we wouldn't encounter cases where CNTVOFF was hopelessly
> ill-configured on a platform, but evidently we have. So we need some
> workaround for that.
>
>> > Yeah, there are a few. The big.LITTLE on the Chromebook 2 models have
>> > this issue, due to the A7 cluster having an incorrect offset
>> > programmed. However, arch timers aren't supported on that SoC in the
>> > first place, so it's not a problem in reality.
>> >
>> > The other known platform is rk3288. It has products out in the wild
>> > where firmware updates are unlikely.
>>
>> One other reason is that (I'm told) that the virtual offset is lost in
>> certain power down conditions (powering down a core, going into S3,
>> etc).  When we power back up the offset is effectively reset to a
>> random value.  That means we need something to reprogram the virtual
>> timer offset whenever we power things back up.
>>
>> If we've got a hypervisor then the hypervisor will definitely be
>> involved in powering things back up and it can reset the virtual
>> offset.  ...but forcing systems to implement a hypervisor (or somehow
>> adding an interface for the kernel to call back into firmware) is a
>> huge effort and it means more hard-to-update code sitting in firmware.
>
> Not if you boot Linux at hyp, as we've recommended for this precise
> reason. That doesn't fix other things like CNTFRQ if the secure
> initialisation doesn't poke that, however.

That's interesting, we could look into that.

> Thanks,
> Mark.
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