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

[Fu Wei]

Hi Mark,

On 1 February 2017 at 02:49, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> On Wed, Feb 01, 2017 at 02:43:02AM +0800, Fu Wei wrote:
>> On 31 January 2017 at 01:49, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> > On Thu, Jan 26, 2017 at 01:49:03PM +0800, Fu Wei wrote:
>> >> On 26 January 2017 at 01:25, Mark Rutland <mark.rutland@xxxxxxx> wrote:
>> >> > On Wed, Jan 25, 2017 at 02:46:12PM +0800, Fu Wei wrote:
>> >> >> 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>
>> >
>> >> But according to another document(ARMv8-A Foundation Platform User
>> >> Guide  ARM DUI0677K),
>> >> Table 3-2 ARMv8-A Foundation Platform memory map (continued)
>> >>
>> >> AP_REFCLK CNTBase0, Generic Timer 64KB   S
>> >> AP_REFCLK CNTBase1, Generic Timer 64KB   S/NS
>> >>
>> >> Dose it means the timer frame 0 can be accessed in SECURE status  only,
>> >> and the timer frame 1 can be accessed in both status?
>> >
>> > That does appear to be what it says.
>> >
>> > I assume in this case CNTCTLBase.CNTSAR<0> is RES0.
>> >
>> >> And because Linux kernel is running on Non-secure EL1, so should we
>> >> skip "SECURE" timer in Linux?
>> >
>> > I guess you mean by checking the GTx Common flags, to see if the timer
>> > is secure? Yes, we must skip those.
>>
>> Yes, exactly.
>>
>> I think we can check the  GTx Common flags, if the timer is set as
>> SECURE, this driver should just skip this timer.
>
> I completely agree that we must skip these.
>
>> > Looking further at this, the ACPI spec is sorely lacking any statement
>> > as to the configuration of CNTCTLBase.{CNTSAR,CNTTIDR,CNTACR}, so it's
>> > not clear if we can access anything in a frame, even if it is listed as
>> > being a non-secure timer.
>> >
>> > I think we need a stronger statement here. Otherwise, we will encounter
>> > problems. Linux currently assumes that CNTCTLBase.CNTACR<N> is
>> > writeable, given a non-secure frame N. This is only the case if
>> > CNTCTLBase.CNTSAR.NS<N> == 1.
>>
>> the original driver has checked these registers, but the problem is:
>> What if the timer frame is designed to be a secure timer, all the
>> register in this frame is only can be accessed in secure status, just
>> like foundation model?
>> Note: for foundation model, Please check Table 3-1 Access permissions
>> of 3.1 ARMv8-A Foundation Platform memory map in ARMv8-A Foundation
>> Platform User Guide
>>
>> So I think we should check the GTDT first, if it's not a secure timer,
>> then we can go on checking CNTSAR. :-)
>
> I've clearly confused matters here. I completely agree that we must skip
> timers the GTDT descrbies as secure.

Yes, got it :-)

>
> My complaint here is that the spec does not explicitly state that
> CNTCTLBase.CNTSAR.NS<N> must be set for timers *not* marked as secure
> (though I believe that is the intent). That is a spec issue, not a code
> issue.

agree :-)

>
> We unfortunately can't check CNTNSAR, as it is secure-only. :(

yes, the spec says:
In a system that implements both Secure and Non-secure states, this
register is only accessible by Secure accesses.

So I think the firmware(from vendor) can decide which timer frame
should be marked as secure according to the GTDT, then kernel just get
this info from GTDT instead of checking CNTNSAR.


>
> Thanks,
> Mark.



-- 
Best regards,

Fu Wei
Software Engineer
Red Hat