Re: [PATCH 1/1] i915/query: Correlate engine and cpu timestamps with better accuracy

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On Sat, May 01, 2021 at 10:27:03AM -0500, Jason Ekstrand wrote:
  On April 30, 2021 23:01:44 "Dixit, Ashutosh" <ashutosh.dixit@xxxxxxxxx>
  wrote:

    On Fri, 30 Apr 2021 19:19:59 -0700, Umesh Nerlige Ramappa wrote:

      On Fri, Apr 30, 2021 at 07:35:41PM -0500, Jason Ekstrand wrote:

        On April 30, 2021 18:00:58 "Dixit, Ashutosh"
        <ashutosh.dixit@xxxxxxxxx>
        wrote:
        On Fri, 30 Apr 2021 15:26:09 -0700, Umesh Nerlige Ramappa wrote:
        Looks like the engine can be dropped since all timestamps are in
        sync.
        I
        just have one more question here. The timestamp itself is 36 bits.
         Should
        the uapi also report the timestamp width to the user OR should I
        just
        return the lower 32 bits of the timestamp?
        Yeah, I think reporting the timestamp width is a good idea since
        we're
        reporting the period/frequency here.

      Actually, I forgot that we are handling the overflow before returning
      the
      cs_cycles to the user and overflow handling was the only reason I
      thought
      user should know the width. Would you stil recommend returning the
      width in
      the uapi?

    The width is needed for userspace to figure out if overflow has occured
    between two successive query calls. I don't think I see this happening
    in
    the code.

  Right... We (UMDs) currently just hard-code it to 36 bits because that's
  what we've had on all platforms since close enough to forever. We bake in
  the frequency based on PCI ID. Returning the number of bits, like I said,
  goes nicely with the frequency. It's not necessary, assuming sufficiently
  smart userspace (neither is frequency), but it seems to go with it. I
  guess I don't care much either way.
  Coming back to the multi-tile issue we discussed internally, I think that
  is something we should care about. Since this works by reading the
  timestamp register on an engine, I think leaving the engine specifier in
  there is fine. Userspace should know that there's actually only one clock
  and just query one of them (probably RCS). For crazy multi-device cases,
  we'll either query per logical device (read tile) or we'll have to make
  them look like a single device and sync the timestamps somehow in the UMD
  by carrying around an offset factor.
  As is, this patch is
  Reviewed-by: Jason Ekstrand <jason@xxxxxxxxxxxxxx>

Thanks, I will add the width here and post the final version.

Regards,
Umesh


  I still need to review the ANV patch before we can land this though.
  --Jason
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