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

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Date:	Thu, 11 Sep 2014 11:58:03 -0400
From:	Christopher Covington <cov@...eaurora.org>
To:	Doug Anderson <dianders@...omium.org>
CC:	olof@...om.net, mark.rutland@....com, devicetree@...r.kernel.org,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>, pawel.moll@....com,
	ijc+devicetree@...lion.org.uk, Marc Zyngier <marc.zyngier@....com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, robh+dt@...nel.org,
	Catalin Marinas <Catalin.Marinas@....com>,
	galak@...eaurora.org, Thomas Gleixner <tglx@...utronix.de>,
	Sonny Rao <sonnyrao@...omium.org>,
	linux-arm-kernel@...ts.infradead.org,
	Nathan Lynch <Nathan_Lynch@...tor.com>
Subject: Re: [PATCH] clocksource: arch_timer: Allow the device tree to specify
 the physical timer

Hi Doug,

On 09/11/2014 11:52 AM, 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.
> 
> 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.
> 
> 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>
> ---
>  Documentation/devicetree/bindings/arm/arch_timer.txt | 6 ++++++
>  drivers/clocksource/arm_arch_timer.c                 | 3 +++
>  2 files changed, 9 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.
> +
> +
>  Example:
>  
>  	timer {
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 5163ec1..8ca07a9 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -649,6 +649,9 @@ static void __init arch_timer_init(struct device_node *np)
>  		arch_timer_ppi[i] = irq_of_parse_and_map(np, i);
>  	arch_timer_detect_rate(NULL, np);
>  
> +	if (of_property_read_bool(np, "arm,use-physical-timer"))
> +		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
> 

How's the VDSO supposed to deal with this? It currently does:

cycle_now = arch_counter_get_cntvct()

Christopher

-- 
Employee of Qualcomm Innovation Center, Inc.
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