[Intel-gfx] [PATCH] i915/perf: Avoid reading OA reports before they land

[Umesh Nerlige Ramappa]

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, occassionally, 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.

Set the pollin only in the else condition in oa_buffer_check_unlocked.

Bug: https://gitlab.freedesktop.org/drm/intel/-/issues/7484
Bug: https://gitlab.freedesktop.org/drm/intel/-/issues/7757
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
---
 drivers/gpu/drm/i915/i915_perf.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
index 19d5652300ee..61536e3c4ac9 100644
--- a/drivers/gpu/drm/i915/i915_perf.c
+++ b/drivers/gpu/drm/i915/i915_perf.c
@@ -545,7 +545,7 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
 	u32 gtt_offset = i915_ggtt_offset(stream->oa_buffer.vma);
 	int report_size = stream->oa_buffer.format->size;
 	unsigned long flags;
-	bool pollin;
+	bool pollin = false;
 	u32 hw_tail;
 	u64 now;
 	u32 partial_report_size;
@@ -620,10 +620,10 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
 		stream->oa_buffer.tail = gtt_offset + tail;
 		stream->oa_buffer.aging_tail = gtt_offset + hw_tail;
 		stream->oa_buffer.aging_timestamp = now;
-	}
 
-	pollin = OA_TAKEN(stream->oa_buffer.tail - gtt_offset,
-			  stream->oa_buffer.head - gtt_offset) >= report_size;
+		pollin = OA_TAKEN(stream->oa_buffer.tail - gtt_offset,
+				  stream->oa_buffer.head - gtt_offset) >= report_size;
+	}
 
 	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
 
-- 
2.38.1