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

From: Daniel Lezcano <daniel.lezcano@linaro.org>
To: Sonny Rao <sonnyrao@chromium.org>, linux-arm-kernel@lists.infradead.org
Cc: devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	Doug Anderson <dianders@chromium.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Olof Johansson <olof@lixom.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Will Deacon <will.deacon@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Sudeep Holla <Sudeep.Holla@arm.com>,
	Mark Rutland <Mark.Rutland@arm.com>,
	Stephen Boyd <sboyd@codeaurora.org>,
	Marc Zyngier <marc.zyngier@arm.com>,
	pawel.moll@arm.com, ijc+devicetree@hellion.org.uk,
	galak@codeaurora.org, Nathan Lynch <Nathan_Lynch@mentor.com>,
	robh+dt@kernel.org
Subject: Re: [PATCH v4] clocksource: arch_timer: Allow the device tree to specify uninitialized timer registers
Date: Wed, 26 Nov 2014 13:49:42 +0100	[thread overview]
Message-ID: <5475CC66.6080600@linaro.org> (raw)
In-Reply-To: <1412753627-28287-1-git-send-email-sonnyrao@chromium.org>

On 10/08/2014 09:33 AM, Sonny Rao wrote:
> From: Doug Anderson <dianders@chromium.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@chromium.org>
> Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>

I would be nice to have Catalin's ack.

Thanks

   -- Daniel

> ---
> 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
> ---
>   Documentation/devicetree/bindings/arm/arch_timer.txt | 8 ++++++++
>   drivers/clocksource/arm_arch_timer.c                 | 8 ++++++++
>   2 files changed, 16 insertions(+)
>
> diff --git a/Documentation/devicetree/bindings/arm/arch_timer.txt b/Documentation/devicetree/bindings/arm/arch_timer.txt
> index 37b2caf..256b4d8 100644
> --- a/Documentation/devicetree/bindings/arm/arch_timer.txt
> +++ b/Documentation/devicetree/bindings/arm/arch_timer.txt
> @@ -22,6 +22,14 @@ 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,cpu-registers-not-fw-configured : Firmware does not initialize
> +  any of the generic timer CPU registers, which contain their
> +  architecturally-defined reset values. Only supported for 32-bit
> +  systems which follow the ARMv7 architected reset values.
> +
> +
>   Example:
>
>   	timer {
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 8daf056..799139f 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -654,6 +654,14 @@ static void __init arch_timer_init(struct device_node *np)
>   	arch_timer_detect_rate(NULL, np);
>
>   	/*
> +	 * If we cannot rely on firmware initializing the timer registers then
> +	 * we should use the physical timers instead.
> +	 */
> +	if (IS_ENABLED(CONFIG_ARM) &&
> +	    of_property_read_bool(np, "arm,cpu-registers-not-fw-configured"))
> +			arch_timer_use_virtual = false;
> +
> +	/*
>   	 * If HYP mode is available, we know that the physical timer
>   	 * has been configured to be accessible from PL1. Use it, so
>   	 * that a guest can use the virtual timer instead.
>

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