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

Fu Wei
2017-01-25T06:46:12Z

Hi Mark,

On 25 January 2017 at 01:24, Mark Rutland <mark.rutland@arm.com> wrote:
> On Wed, Jan 18, 2017 at 09:25:32PM +0800, fu.wei@linaro.org wrote:
>> From: Fu Wei <fu.wei@linaro.org>
>>
>> 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)

So I guess the logic of the original code is
static u32 arch_timer_rate keeps the frequency of the system counter,
no matter where the value comes from.
Because they should be the same value. if we have got the frequency
of the system counter(arch_timer_rate != 0), then we don't need to get
it again, even in anther way.

But the above is just my thought, and I believe you're the expert of
ARM. So please correct me if I misunderstand something. :-)

Thanks!
>
> The probing logic relying on this is complicated and fragile, and I
> think these patches are complicating that further (though I appreciate
> that's far from the intent).
>
> I believe we need to split the MMIO and sysreg timer code apart
> entirely. I've had a look at that today, though it's been fairly painful
> so far. It appears some platforms may inadvertently be relying on the
> order and manner in which the rates are probed, which is a major
> headache.
>
> I will try to attack that some more tomorrow.
>
>> This patch separates paths to determine frequency:
>> Separate out the MMIO frequency and the sysreg frequency detection call,
>> and use the appropriate one for the counter.
>>
>> Signed-off-by: Fu Wei <fu.wei@linaro.org>
>> ---
>> drivers/clocksource/arm_arch_timer.c | 40 ++++++++++++++++++++++--------------
>> 1 file changed, 25 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
>> index 6484f84..9482481 100644
>> --- a/drivers/clocksource/arm_arch_timer.c
>> +++ b/drivers/clocksource/arm_arch_timer.c
>> @@ -488,23 +488,33 @@ static int arch_timer_starting_cpu(unsigned int cpu)
>> return 0;
>> }
>>
>> -static void arch_timer_detect_rate(void __iomem *cntbase)
>> +static void __arch_timer_determine_rate(u32 rate)
>> {
>> - /* Who has more than one independent system counter? */
>> - if (arch_timer_rate)
>> - return;
>> + /* Check the timer frequency. */
>> + if (!arch_timer_rate) {
>> + if (rate)
>> + arch_timer_rate = rate;
>> + else
>> + pr_warn("frequency not available\n");
>> + } else if (rate && arch_timer_rate != rate) {
>> + pr_warn("got different frequency, keep original.\n");
>> + }
>> +}
>
> This function should be killed off entirely. We need to be able to fail
> the probe if we cannot determine the rate, and that means we need error
> handling in the ACPI and DT cases anyway.
>
> Thanks,
> Mark.

--
Best regards,

Fu Wei
Software Engineer
Red Hat