Re: [PATCH v20 08/17] clocksource/drivers/arm_arch_timer: Rework counter frequency detection.

From: Christopher Covington
Date: Wed Jan 25 2017 - 10:38:22 EST

Hi Fu,

On 01/25/2017 01:46 AM, Fu Wei wrote:
> Hi Mark,
> On 25 January 2017 at 01:24, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> On Wed, Jan 18, 2017 at 09:25:32PM +0800, fu.wei@xxxxxxxxxx wrote:
>>> From: Fu Wei <fu.wei@xxxxxxxxxx>
>>> The counter frequency detection call(arch_timer_detect_rate) combines two
>>> ways to get counter frequency: system coprocessor register and MMIO timer.
>>> But in a specific timer init code, we only need one way to try:
>>> getting frequency from MMIO timer register will be needed only when we
>>> init MMIO timer; getting frequency from system coprocessor register will
>>> be needed only when we init arch timer.
>> When I mentioned this splitting before, I had mean that we'd completely
>> separate the two, with separate mmio_rate and sysreg_rate variables.
> sorry for misunderstanding.
> Are you saying :
> diff --git a/drivers/clocksource/arm_arch_timer.c
> b/drivers/clocksource/arm_arch_timer.c
> index 663a57a..eec92f6 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -65,7 +65,8 @@ struct arch_timer {
> #define to_arch_timer(e) container_of(e, struct arch_timer, evt)
> -static u32 arch_timer_rate;
> +static u32 arch_timer_sysreg_rate ;
> +static u32 arch_timer_mmio_rate;
> static int arch_timer_ppi[ARCH_TIMER_MAX_TIMER_PPI];
> static struct clock_event_device __percpu *arch_timer_evt;
> But what have I learned From ARMv8 ARM is
> AArch64 System register CNTFRQ_EL0 is provided so that software can
> discover the frequency of the system counter.
> CNTFRQ(in CNTCTLBase and CNTBaseN) is provided so that software can
> discover the frequency of the system counter.
> The bit assignments of the registers are identical in the System
> register interface and in the memory-mapped system level interface.
> So I think they both contain the same value : the frequency of the
> system counter, just in different view, and can be accessed in
> different ways.
> So do we really need to separate mmio_rate and sysreg_rate variables?
> And for CNTFRQ(in CNTCTLBase and CNTBaseN) , we can NOT access it in
> Linux kernel (EL1),
> Because ARMv8 ARM says:
> In a system that implements both Secure and Non-secure states, this
> register is only accessible by Secure accesses.
> That means we still need to get the frequency of the system counter
> from CNTFRQ_EL0 in MMIO timer code.
> This have been proved when I tested this driver on foundation model, I
> got "0" when I access CNTFRQ from Linux kernel (Non-secure EL1)

That sounds like a firmware problem. Firmware in EL3 is supposed to write
the value into CNTFRQ. If you're not currently using any firmware, I'd
recommend the bootwrapper on models/simulators/emulators.


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