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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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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