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

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Date:	Thu, 4 Dec 2014 23:34:25 -0800
From:	Olof Johansson <olof@...om.net>
To:	Sonny Rao <sonnyrao@...omium.org>
Cc:	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Doug Anderson <dianders@...omium.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Marc Zyngier <marc.zyngier@....com>, pawel.moll@....com,
	ijc+devicetree@...lion.org.uk, galak@...eaurora.org,
	Nathan Lynch <Nathan_Lynch@...tor.com>, robh+dt@...nel.org
Subject: Re: [PATCH v4] clocksource: arch_timer: Allow the device tree to
 specify uninitialized timer registers

On Wed, Oct 08, 2014 at 12:33:47AM -0700, 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>
> ---
> Changes in v2:
> - Add "#ifdef CONFIG_ARM" as per Will Deacon
> 
> Changes in v3:
> - change property name to arm,cntvoff-not-fw-configured and specify
>   that the value of CNTHCTL.PL1PC(T)EN must still be the reset value
>   of 1 as per Mark Rutland
> 
> Changes in v4:
> - change property name to arm,cpu-registers-not-fw-configured and
>   specify that all cpu registers must have architected reset values
>   per Mark Rutland
> - change from "#ifdef CONFIG_ARM" to "if (IS_ENABLED(CONFIG_ARM))" per
>   Arnd Bergmann


Applied to next/drivers (and next/dt for rk3288 dependency). Thanks, all!


-Olof
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