From 4155fc07fa9fd691d424e7f8fb64591cccb88788 Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Fri, 20 Mar 2015 17:57:47 +0000 Subject: 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 it is exposed to all software in a privileged non-secure mode (including guests running under a hypervisor). The firmware and DTs for recent SoCs have followed 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 Acked-by: Catalin Marinas Acked-by: Marc Zyngier Acked-by: Olof Johansson Acked-by: Stephen Boyd Cc: Rob Herring Cc: Will Deacon Signed-off-by: Rob Herring --- Documentation/devicetree/bindings/arm/arch_timer.txt | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) (limited to 'Documentation/devicetree/bindings/arm/arch_timer.txt') diff --git a/Documentation/devicetree/bindings/arm/arch_timer.txt b/Documentation/devicetree/bindings/arm/arch_timer.txt index 256b4d8..e774128 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. @@ -46,7 +49,8 @@ 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. -- cgit v1.1