linux-kernel - Re: [PATCH v4] clocksource: arch_timer: Allow the device tree to specify uninitialized timer registers

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Message-ID: <20141128141648.GB4249@localhost>
Date:	Fri, 28 Nov 2014 14:16:48 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	Sonny Rao <sonnyrao@...omium.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Doug Anderson <dianders@...omium.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Olof Johansson <olof@...om.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Pawel Moll <Pawel.Moll@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	Nathan Lynch <Nathan_Lynch@...tor.com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>
Subject: Re: [PATCH v4] clocksource: arch_timer: Allow the device tree to
 specify uninitialized timer registers

On Wed, Nov 26, 2014 at 12:49:42PM +0000, Daniel Lezcano wrote:
> On 10/08/2014 09:33 AM, Sonny Rao wrote:
> > From: Doug Anderson <dianders@...omium.org>
> >
> > Some 32-bit (ARMv7) systems are architected like this:
> >
> > * The firmware doesn't know and doesn't care about hypervisor mode and
> >    we don't want to add the complexity of hypervisor there.
> >
> > * The firmware isn't involved in SMP bringup or resume.
> >
> > * The ARCH timer come up with an uninitialized offset (CNTVOFF)
> >    between the virtual and physical counters.  Each core gets a
> >    different random offset.
> >
> > * The device boots in "Secure SVC" mode.
> >
> > * Nothing has touched the reset value of CNTHCTL.PL1PCEN or
> >    CNTHCTL.PL1PCTEN (both default to 1 at reset)
> >
> > On systems like the above, it doesn't make sense to use the virtual
> > counter.  There's nobody managing the offset and each time a core goes
> > down and comes back up it will get reinitialized to some other random
> > value.
> >
> > This adds an optional property which can inform the kernel of this
> > situation, and firmware is free to remove the property if it is going
> > to initialize the CNTVOFF registers when each CPU comes out of reset.
> >
> > Currently, the best course of action in this case is to use the
> > physical timer, which is why it is important that CNTHCTL hasn't been
> > changed from its reset value and it's a reasonable assumption given
> > that the firmware has never entered HYP mode.
> >
> > Note that it's been said that on ARMv8 systems the firmware and
> > kernel really can't be architected as described above.  That means
> > using the physical timer like this really only makes sense for ARMv7
> > systems.
> >
> > Signed-off-by: Doug Anderson <dianders@...omium.org>
> > Signed-off-by: Sonny Rao <sonnyrao@...omium.org>
> > Reviewed-by: Mark Rutland <mark.rutland@....com>
> 
> Acked-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> 
> I would be nice to have Catalin's ack.

FWIW:

Acked-by: Catalin Marinas <catalin.marinas@....com>
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