On Thu, Aug 28, 2014 at 06:27:04PM +0100, Marc Zyngier wrote:

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

quoted

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.

True.

s/KVM/KVM\/Xen/, then?

quoted

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 <redacted>

Short of more explicit threats:

Acked-by: Marc Zyngier <redacted>

Cheers.

Mark.

quoted

---
 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.
 

-- 
Jazz is not dead. It just smells funny...