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

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Date:	Thu, 11 Sep 2014 17:47:10 +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 05:16:44PM +0100, Doug Anderson wrote:
> 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 between the
>   virtual and physical counters.  Each core gets a different random
>   offset.
> 
> 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.

You probably need to rephrase this slightly, as there *is* still a
requirement on the hypervisor/firmware (actually, two!). See below.

> Let's add a property to the device tree to say that we shouldn't use
> the virtual timer.  Firmware could potentially remove this property
> before passing the device tree to the kernel if it really wants the
> kernel to use a virtual timer.
> 
> Note that it's been said that ARM64 (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.

I'd go further: this only makes sense if you're booting in secure SVC
mode.

> In order for this patch to do anything useful, we also need Sonny's
> patch at <https://patchwork.kernel.org/patch/4790921/>
> 
> Signed-off-by: Doug Anderson <dianders@...omium.org>
> Signed-off-by: Sonny Rao <sonnyrao@...omium.org>
> ---
> Changes in v2:
> - Add "#ifdef CONFIG_ARM" as per Will Deacon
> 
>  Documentation/devicetree/bindings/arm/arch_timer.txt | 6 ++++++
>  drivers/clocksource/arm_arch_timer.c                 | 5 +++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/arm/arch_timer.txt b/Documentation/devicetree/bindings/arm/arch_timer.txt
> index 37b2caf..876d32b 100644
> --- a/Documentation/devicetree/bindings/arm/arch_timer.txt
> +++ b/Documentation/devicetree/bindings/arm/arch_timer.txt
> @@ -22,6 +22,12 @@ to deliver its interrupts via SPIs.
>  - always-on : a boolean property. If present, the timer is powered through an
>    always-on power domain, therefore it never loses context.
>  
> +** Optional properties:
> +
> +- arm,use-physical-timer : Don't ever use the virtual timer, just use the
> +  physical one.  Not supported for ARM64.

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).

Will
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