linux-kernel - Re: [PATCH v2] clocksource: arch_timer: Allow the device tree to specify the physical timer

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Message-ID: <20140911170716.GA6158@arm.com>
Date:	Thu, 11 Sep 2014 18:07:16 +0100
From:	Will Deacon <will.deacon@....com>
To:	Doug Anderson <dianders@...omium.org>
Cc:	"olof@...om.net" <olof@...om.net>,
	Sonny Rao <sonnyrao@...omium.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Christopher Covington <cov@...eaurora.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Nathan Lynch <Nathan_Lynch@...tor.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] clocksource: arch_timer: Allow the device tree to
 specify the physical timer

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@....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
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