Doc: dt: arch_timer: discourage clock-frequency use
PATCH RESEND 1/1 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.
--- Documentation/devicetree/bindings/arm/arch_timer.txt | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
This patch has fallen through the cracks a few times [1,2], though that appears to be my fault for not sorting out a reasonable Cc list.
Rob, are you happy to take this through your tree?
Mark.
[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/282804.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/282830.html
Documentation/devicetree/bindings/arm/arch_timer.txt +6 -2
@@ -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.
Signature · 1 line
1.9.1
PATCH RESEND 1/1 Doc: dt: arch_timer: discourage clock-frequency use
Quoted 26 lines · Mark Rutland
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 <[email protected]>Acked-by Catalin Marinas <[email protected]>Acked-by Marc Zyngier <[email protected]>Acked-by Olof Johansson <[email protected]>Acked-by Stephen Boyd <[email protected]>Cc Rob Herring <[email protected]>Cc Will Deacon <[email protected]>
Quoted 8 lines
--- Documentation/devicetree/bindings/arm/arch_timer.txt | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)This patch has fallen through the cracks a few times [1,2], though that appears to be my fault for not sorting out a reasonable Cc list.
Rob, are you happy to take this through your tree?
Yes, I'll take it.
Rob
Quoted 36 lines
Mark.
[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/282804.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/282830.htmlDocumentation/devicetree/bindings/arm/arch_timer.txt +6
-3@@ -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. --1.9.1
PATCH RESEND 1/1 Doc: dt: arch_timer: discourage clock-frequency use
Quoted 3 lines
Quoted 1 line
Rob, are you happy to take this through your tree?
Yes, I'll take it.
Are you still planning on taking this for v4.1? I couldn't spot it in a public branch yet.
Mark
Quoted 40 lines
Rob
Quoted 36 lines
Mark.
[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/282804.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/282830.htmlDocumentation/devicetree/bindings/arm/arch_timer.txt +6
-3@@ -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. --1.9.1
PATCH RESEND 1/1 Doc: dt: arch_timer: discourage clock-frequency use
Quoted 6 lines · Mark Rutland
Quoted 3 lines
Quoted 1 line
Rob, are you happy to take this through your tree?
Yes, I'll take it.
Are you still planning on taking this for v4.1? I couldn't spot it in a public branch yet.
Humm, I had marked that I applied it, but you're right. Anyway, it is there now in my for-next branch. Sorry about that.
Rob
PATCH RESEND 1/1 Doc: dt: arch_timer: discourage clock-frequency use
Quoted 10 lines · Rob Herring
Quoted 6 lines · Mark Rutland
Quoted 3 lines
Quoted 1 line
Rob, are you happy to take this through your tree?
Yes, I'll take it.
Are you still planning on taking this for v4.1? I couldn't spot it in a public branch yet.
Humm, I had marked that I applied it, but you're right. Anyway, it is there now in my for-next branch. Sorry about that.
No worries. Thanks for picking it up!
Mark.