[Intel-gfx] [PATCH] drm/i915/perf: Wrap 64bit divides in do_div()

[Robert Bragg]

On Nov 22, 2016 23:49, "Chris Wilson" <chris at chris-wilson.co.uk> wrote:
>
> On Tue, Nov 22, 2016 at 11:32:38PM +0000, Robert Bragg wrote:
> >    Thanks for sending out. It looked good to me, but testing shows a
'divide
> >    error'.
> >
> >    I haven't double checked, but I think it's because the max OA
exponent
> >    (31) converted to nanoseconds is > UINT32_MAX with the lower 32bits
zero
> >    and the do_div denominator argument is only 32bit.
>
> Hmm, I thought do_div() was u64 / u64, but no it is u64 / u32. Looks
> like the appropriate function would be div64_u64().
>
> >    It corresponds to a 5 minute period which is a bit silly, so we could
> >    reduce the max exponent. A period of UINT32_MAX is about 4 seconds
where I
> >    can't currently think of a good use case for such a low frequency.
> >
> >    Instead of changing the max OA exponent (where the relationship to
the
> >    period changes for gen9 and may become fuzzy if we start training
our view
> >    of the gpu timestamp frequency instead of using constants) maybe we
should
> >    set an early limit on an exponent resulting in a period > UINT32_MAX?
>
> Seems like picking the right function would help!

Or that, yep. Sounds good to me, thanks.
- Robert

> -Chris
>
> --
> Chris Wilson, Intel Open Source Technology Centre
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