drm/i915/perf: improve tail race workaround

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.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>