Use high-precision vblank timings, if available #33
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added 4 commits
August 9, 2019 17:53
When reading drm_vblank_event events, parse the following optional fields: - 'time': a possibly more accurate timestamp better representing the actual HW vblank timestamp - 'high_prec': true if 'time' is a high-precision HW timestamp from a drm driver with this particular capability Fields are only parsed if the ftrace clock was set to `mono`, otherwise the timestamps can't be aligned in a meaningful way. With ftrace, the tracing clock can be set to `mono like this: `echo mono > /sys/kernel/tracing/trace_clock` Also add helper functions to retrieve the high-precision timestamp.
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Hi Heinrich. This looks good to me. Let me know when you want to have it pulled and I'll get it in. Thanks much. |
hfink-daqri
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The kernel patch has been accepted and is currently in drm-misc-next. It's supposed to land for 5.5. I think this MR is ready to be merged. |
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It's in. Thanks Heinrich! |
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This (work-in-progress) PR implements the use of more accurate vblank timestamps, as suggested in #30 :
It requires a small kernel patch (submitted to the mailing list here), that extends the
drm_vblank_eventwith two fields:timeholding a monotonic clock timestamp, andhigh_prec, a boolean indicating whether or not the timestamp came from a DRM driver that is able to generate HW-corrected high-precision vblank timestamps, or not.GPUVis is fully backwards-compatible to how it worked before. High-precision vblank timestamps are only used in GPUVis if the following is true:
OPT_VBlankHighPrecTimestamps, default is OFF) (bound to Ctrl-Shift-K, or right-click to toggle in option pop-up)timefield is set in thedrm_vblank_eventdata, andhigh_precis trueOpening an enriched trace, you can now toggle back and forth using Ctrl-Shift-K and see how the purple lines jump back and forth (on my laptop, diff is ~ 200ms).
As described above, you need to configure ftrace to use monotonic clock as the source for timestamps:
echo mono > /sys/kernel/tracing/trace_clockWithout that, we can't easily align the timestamps of the vblank event along the ftrace/GPUVis timeline.