On 28/08/14 18:03, Mark Rutland wrote: > From 67104ad5a56e4c18f9c41f06af028b7561740afd Mon Sep 17 00:00:00 2001 > From: Mark Rutland <mark.rutland@xxxxxxx> > 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@xxxxxxx> > Cc: Marc Zyngier <marc.zyngier@xxxxxxx> Short of more explicit threats: Acked-by: Marc Zyngier <marc.zyngier@xxxxxxx> > --- > 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... -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
