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

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Date:	Wed, 27 Aug 2014 19:58:53 -0700
From:	Olof Johansson <olof@...om.net>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Sonny Rao <sonnyrao@...omium.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Russell King <linux@....linux.org.uk>,
	Sudeep KarkadaNagesha <sudeep.karkadanagesha@....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>,
	Doug Anderson <dianders@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <Mark.Rutland@....com>
Subject: Re: [PATCH] clocksource: arch_timer: Fix code to use physical timers
 when requested

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?

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.

Essentially, I expect many vendors who use BSP kernels by default to
have firmware that forgets to setup the offset, since hardware doesn't
come up with a default one, and their older BSP kernels doesn't access
the virtual one.

> In this particular case is there actually a virtual interrupt but we've
> explicitly removed it from the DT so that the driver can be forced into
> using the physical counter? Or are we getting saved by the hyp check?

The SoC has the virtual timer, and if it has firmware that supports it
there's a good reason to still have it there. After all, DT describes
hardware.

I have a patch I should post that adds a property to make the driver
pick the physical timer instead, since right now it'll always use
virtual if it's available. I'll try to get that posted later tonight.

-Olof
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