[PATCH v3 3/3] arm64: arch_timer: Work around QorIQ Erratum A-008585

[oss@buserror.net (Scott Wood)]

On Thu, 2016-07-07 at 20:59 +0800, Ding Tianhong wrote:
> On 2016/7/7 19:51, Marc Zyngier wrote:
> > 
> > On 07/07/16 12:37, Ding Tianhong wrote:
> > > 
> > > On 2016/7/7 17:49, Marc Zyngier wrote:
> > > > 
> > > > What makes you think that ignoring the two bottom bits is a safe thing
> > > > to do? Talking about performance when the HW has such a dramatic bug
> > > > is
> > > > like putting a bigger engine on a car that has no brakes: you just hit
> > > > the wall quicker.
> > > > 
> > > > Thanks,
> > > > 
> > > I have a chip which has the same problem like Scott's chip, and I
> > > wish to solve this problem in the same way, our chip designer told me
> > > that if you got a wrong value from the cntvct_el0, you would not get
> > > a wrong value until 8 cycles later, so I could ignoring the lowest 3
> > > bits if I reading twice together.
> > Is that CPU cycles? Or timer cycles? What guarantees do you have that
> > the two reads are *always* done in the right timing window?
> > 
> The timer counter only use 56 bits in aarch64, my chip would change one of
> the higher?
> bit(55 to 3) to a wrong value when occur bug, so there will be more than 8
> cycles between
> correct value and wrong value from the timer counter. Maybe Scott's problem
> is not just like
> mine.

It's not like yours. ?Most errors I saw were time going backwards by 1, 3, or
7 cycles (with occasional larger errors).

-Scott