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

[Chris Wilson]

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!
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre