From mboxrd@z Thu Jan  1 00:00:00 1970
From: Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v2] clocksource: arch_timer: Allow the device tree to
 specify the physical timer
Date: Thu, 11 Sep 2014 18:07:16 +0100
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To: Doug Anderson <dianders@chromium.org>
Cc: "olof@lixom.net" <olof@lixom.net>, Sonny Rao <sonnyrao@chromium.org>, Catalin Marinas <Catalin.Marinas@arm.com>, Mark Rutland <Mark.Rutland@arm.com>, Stephen Boyd <sboyd@codeaurora.org>, Marc Zyngier <Marc.Zyngier@arm.com>, Sudeep Holla <Sudeep.Holla@arm.com>, Christopher Covington <cov@codeaurora.org>, Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>, Thomas Gleixner <tglx@linutronix.de>, Daniel Lezcano <daniel.lezcano@linaro.org>, Nathan Lynch <Nathan_Lynch@mentor.com>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, "robh+dt@kernel.org" <robh+dt@kernel.org>, Pawel Moll <Pawel.Moll@arm.com>, "ijc+devicetree@hellion.org.uk" <ijc+devicetree@hellion.org.uk>, "galak@codeaurora.org" <galak@codeaurora.org>, "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Thu, Sep 11, 2014 at 05:59:53PM +0100, Doug Anderson wrote:
> On Thu, Sep 11, 2014 at 9:47 AM, Will Deacon <will.deacon@arm.com> wrote:
> > I'd say `Only supported for ARM' to better match what we've done. Probably
> > also worth mentioning that this relies on the hypervisor/firmware having set
> > CNTHCTL.PL1PCEN and CNTHCTL.EL1PCTEN (but assumedly made a mess of CNTVOFF
> > ;) if you want to boot on the non-secure side (e.g. as a guest).
> 
> Note that the reset value of CNTHCTL.PL1PCEN and CNTHCTL.PL1PCTEN are
> both 1 in my version of the ARM ARM.  On the other hand CNTVOFF is
> documented to have an UNKNOWN reset value.  If only ARM had guaranteed
> that CNTVOFF started out as 0 (which seems like it would have been
> sensible) we wouldn't be in this mess.  :-/

I'm afraid we went the opposite way -- in ARMv8 there are a tiny handful
of EL3 registers that are well-defined out of reset, then the rest of the
system is UNKNOWN. The hardware guys prefer that and it can also be useful
for very low-level debugging (system crashes, do a reset, read out the
state).

Will