i915/perf: Drop the aging_tail logic in perf OA
On DG2, capturing OA reports while running heavy render workloads sometimes results in invalid OA reports where 64-byte chunks inside reports have stale values. Under memory pressure, high OA sampling rates (13.3 us) and heavy render workload, occasionally, the OA HW TAIL pointer does not progress as fast as the sampling rate. When these glitches occur, the TAIL pointer takes approx. 200us to progress. While this is expected behavior from the HW perspective, invalid reports are not expected. In oa_buffer_check_unlocked(), when we execute the if condition, we are updating the oa_buffer.tail to the aging tail and then setting pollin based on this tail value, however, we do not have a chance to rewind and validate the reports prior to setting pollin. The validation happens in a subsequent call to oa_buffer_check_unlocked(). If a read occurs before this validation, then we end up reading reports up until this oa_buffer.tail value which includes invalid reports. Though found on DG2, this affects all platforms. The aging tail logic is no longer necessary since we are explicitly checking for landed reports. Start by dropping the aging tail logic. v2: - Drop extra blank line - Add reason to drop aging logic (Ashutosh) - Add bug links (Ashutosh) - rename aged_tail to read_tail - Squash patches 3 and 1 v3: (Ashutosh) - Remove extra spaces - Remove gtt_offset from the pollin calculation - s/Bug:/Link/ in commit message (checkpatch) Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7484 Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7757Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com> Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20230605193923.1836048-2-umesh.nerlige.ramappa@intel.com
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