Re: [linux-sunxi] Re: [PATCH 0/2] Allwinner A64 timer workaround

[Andre Przywara <andre.przywara@arm.com>]

Hi,

On 04/07/18 16:01, Marc Zyngier wrote:
> On Wed, 04 Jul 2018 15:44:36 +0100,
> Andre Przywara <andre.przywara@arm.com> wrote:
>>
>> Hi,
>>
>> On 04/07/18 15:31, Thomas Gleixner wrote:
>>> On Wed, 4 Jul 2018, Andre Przywara wrote:
>>>> On 04/07/18 11:00, Thomas Gleixner wrote:
>>>>> On Wed, 4 Jul 2018, Marc Zyngier wrote:
>>>>>> On 04/07/18 09:23, Daniel Lezcano wrote:
>>>>>>>
>>>>>>> If the patches fix a bug which already exist, it makes sense to
>>>>>>> propagated the fix back to the stable versions.
>>>>>>
>>>>>> That's your call, but I'm not supportive of that decision, specially as
>>>>>> we have information from the person developing the workaround that this
>>>>>> doesn't fully address the issue.
>>>>>
>>>>> The patches should not be applied at all. Simply because they don't fix the
>>>>> issue completely.
>>>>>
>>>>> From a quick glance at various links and information about this, this very
>>>>> much smells like the FSL_ERRATUM_A008585.
>>>>> Has that been tried? It looks way more robust than the magic 11 bit
>>>>> crystal ball logic.
>>>>
>>>> The Freescale erratum is similar, but not identical [1].
>>>> It seems like the A64 is less variable, so we can use a cheaper
>>>> workaround, which gets away with normally just one sysreg read. But then
>>>> again the newer error reports may actually suggest otherwise ...
>>>>
>>>> And as it currently stands, the Freescale erratum has the drawback of
>>>> relying on the CPU running much faster than the timer. The A64 can run
>>>> at 24 MHz (for power savings, or possibly during DVFS transitions),
>>>> which is the timer frequency. So subsequent counter reads will never
>>>> return the same value and the workaround times out.
>>>
>>> If that's the case then you need to find a different functional timer for
>>> time keeping. Having an erratic behaving timer for time keeping is not an
>>> option at all.
>>
>> That's not an option on arm64. There are other usable time sources in
>> the SoC, but the arch timer is somewhat mandatory for all practical
>> purposes on arm64. We rely on it in some many places that it's not
>> feasible to run without it. That's why we call it "architected" timer
>> after all ;-)
>> But I am quite confident that we can find a correct workaround. Maybe
>> it's really the TVAL (the downcounter) write which is the culprit here,
>> since the hardware actually writes "now() + TVAL" into the CVAL
>> (upcounter) register. This internal counter access may be flawed as well.
> 
> You got it backward: CVAL is not a counter at all. It is a
> Comparator. And TVAL has an implicit read from the counter, as it is
> defined as "CVAL - CNT" (i.e. the number of ticks until the timer
> expires).

Yes, that's what I meant actually, sorry for the lousy wording.

What I am actually more concerned about than reading (do we actually
read TVAL?), is writing TVAL. The original BSP errata hack hints at this
being a problem:
https://github.com/longsleep/linux-pine64/blob/5b10a45ae8b0/drivers/clocksource/arm_arch_timer.c#L231-L244

> So it might be worth trying to handle TVAL entirely in SW.
> 
> But this relies on being able to read the timer and get a number of
> correct values out of it. One possibility would be to sacrifice
> precision and always ignore some of the bottom bits, but this is
> always going to suck terribly.
> 
> The alternative is burn that thing, and pretend it never existed.

Yes, that crossed my mind multiple times.

Cheers,
Andre.