drm/i915/perf: improve tail race workaround
There's a HW race condition between OA unit tail pointer register updates and writes to memory whereby the tail pointer can sometimes get ahead of what's been written out to the OA buffer so far (in terms of what's visible to the CPU). Although this can be observed explicitly while copying reports to userspace by checking for a zeroed report-id field in tail reports, we want to account for this earlier, as part of the _oa_buffer_check to avoid lots of redundant read() attempts. Previously the driver used to define an effective tail pointer that lagged the real pointer by a 'tail margin' measured in bytes derived from OA_TAIL_MARGIN_NSEC and the configured sampling frequency. Unfortunately this was flawed considering that the OA unit may also automatically generate non-periodic reports (such as on context switch) or the OA unit may be enabled without any periodic sampling. This improves how we define a tail pointer for reading that lags the real tail pointer by at least %OA_TAIL_MARGIN_NSEC nanoseconds, which gives enough time for the corresponding reports to become visible to the CPU. The driver now maintains two tail pointers: 1) An 'aging' tail with an associated timestamp that is tracked until we can trust the corresponding data is visible to the CPU; at which point it is considered 'aged'. 2) An 'aged' tail that can be used for read()ing. The two separate pointers let us decouple read()s from tail pointer aging. The tail pointers are checked and updated at a limited rate within a hrtimer callback (the same callback that is used for delivering POLLIN events) and since we're now measuring the wall clock time elapsed since a given tail pointer was read the mechanism no longer cares about the OA unit's periodic sampling frequency. The natural place to handle the tail pointer updates was in gen7_oa_buffer_is_empty() which is called as part of blocking reads and the hrtimer callback used for polling, and so this was renamed to oa_buffer_check() considering the added side effect while checking whether the buffer contains data. Signed-off-by:Robert Bragg <robert@sixbynine.org> Reviewed-by:
Matthew Auld <matthew.auld@intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/20170511154345.962-6-lionel.g.landwerlin@intel.comSigned-off-by:
Chris Wilson <chris@chris-wilson.co.uk>
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