Re: [PATCH 11/14] arm64: dts: Add initial device tree support for EXYNOS7

[Marc Zyngier <marc.zyngier@arm.com>]

On 28/08/14 18:03, Mark Rutland wrote:

> From 67104ad5a56e4c18f9c41f06af028b7561740afd Mon Sep 17 00:00:00 2001
> From: Mark Rutland <mark.rutland@arm.com>
> Date: Thu, 28 Aug 2014 17:41:03 +0100
> Subject: [PATCH] Doc: dt: arch_timer: discourage clock-frequency use
> 
> The ARM Generic Timer (AKA the architected timer, arm_arch_timer)
> features a CPU register (CNTFRQ) which firmware is intended to
> initialize, and non-secure software can read to determine the frequency
> of the timer. On CPUs with secure state, this register cannot be written
> from non-secure states.
> 
> The firmware of early SoCs featuring the timer did not correctly
> initialize CNTFRQ correctly on all CPUs, requiring the frequency to be
> described in DT as a workaround. This workaround is not complete however
> as CNTFRQ is exposed to all software in a privileged non-secure mode,
> including KVM guests. The firmware and DTs for recent SoCs have followed

I believe Xen is also affected by this.

> the example set by these early SoCs.
> 
> This patch updates the arch timer binding documentation to make it
> clearer that the use of the clock-frequency property is a poor
> work-around. The MMIO generic timer binding is similarly updated, though
> this is less of a concern as there is generally no need to expose the
> MMIO timers to guest OSs.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>

Short of more explicit threats:

Acked-by: Marc Zyngier <marc.zyngier@arm.com>

> ---
>  Documentation/devicetree/bindings/arm/arch_timer.txt | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/arm/arch_timer.txt b/Documentation/devicetree/bindings/arm/arch_timer.txt
> index 37b2caf..5ca3f95 100644
> --- a/Documentation/devicetree/bindings/arm/arch_timer.txt
> +++ b/Documentation/devicetree/bindings/arm/arch_timer.txt
> @@ -17,7 +17,10 @@ to deliver its interrupts via SPIs.
>  - interrupts : Interrupt list for secure, non-secure, virtual and
>    hypervisor timers, in that order.
>  
> -- clock-frequency : The frequency of the main counter, in Hz. Optional.
> +- clock-frequency : The frequency of the main counter, in Hz. Should be present
> +  only where necessary to work around BROKEN firmware which does not configure
> +  CNTFRQ on all CPUs to a uniform correct value. Use of this property is
> +  STRONGLY DISCOURAGED; fix your firmware unless absolutely impossible.
>  
>  - always-on : a boolean property. If present, the timer is powered through an
>    always-on power domain, therefore it never loses context.
> @@ -38,7 +41,8 @@ Example:
>  
>  - compatible : Should at least contain "arm,armv7-timer-mem".
>  
> -- clock-frequency : The frequency of the main counter, in Hz. Optional.
> +- clock-frequency : The frequency of the main counter, in Hz. Should be present
> +  only when firmware has not configured the MMIO CNTFRQ registers.
>  
>  - reg : The control frame base address.
>  
> 


-- 
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