[PATCH 3/3] drm/xe/xe_oa: Add a lag to the reports that is exported to user

When running heavy workloads, reading the OA reports too soon does not
guarantee that the report has landed in memory. To make sure correct
reports are copied to user buffer, only return reports that lag the
current HW_TAIL register by 32 reports. This is an empirical number
based on a heavy render workload and several test iterations.

Signed-off-by: Umesh Nerlige Ramappa <[email protected]>
---
v2: Fix the LAG logic by using sliding window (Sashiko)
---
 drivers/gpu/drm/xe/xe_oa.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_oa.c b/drivers/gpu/drm/xe/xe_oa.c
index c7f9c3955cb5..731b56684439 100644
--- a/drivers/gpu/drm/xe/xe_oa.c
+++ b/drivers/gpu/drm/xe/xe_oa.c
@@ -224,7 +224,7 @@ static bool mert_wa_14026633728(struct xe_oa_stream *s)
 static bool xe_oa_buffer_check_unlocked(struct xe_oa_stream *stream)
 {
 	u32 gtt_offset = xe_bo_ggtt_addr(stream->oa_buffer.bo);
-	u32 hw_tail, partial_report_size, available;
+	u32 hw_tail, partial_report_size, available, lag;
 	int report_size = stream->oa_buffer.format->size;
 	unsigned long flags;
 
@@ -234,17 +234,20 @@ static bool xe_oa_buffer_check_unlocked(struct xe_oa_stream *stream)
 	hw_tail -= gtt_offset;
 
 	/*
-	 * The tail pointer increases in 64 byte (cacheline size), not in report_size
+	 * The hw_tail pointer increases in 64 byte (cacheline size), not in report_size
 	 * increments. Also report size may not be a power of 2. Compute potential
 	 * partially landed report in OA buffer.
 	 */
 	partial_report_size = xe_oa_circ_diff(stream, hw_tail, stream->oa_buffer.tail);
 	partial_report_size %= report_size;
 
-	/* Subtract partial amount off the tail */
+	/* Subtract partial amount off the hw_tail */
 	hw_tail = xe_oa_circ_diff(stream, hw_tail, partial_report_size);
 
-	stream->oa_buffer.tail = hw_tail;
+#define LAG_REPORTS 32
+	lag = xe_oa_circ_diff(stream, hw_tail, stream->oa_buffer.tail);
+	if (lag > LAG_REPORTS * report_size)
+		stream->oa_buffer.tail = xe_oa_circ_diff(stream, hw_tail, LAG_REPORTS * report_size);
 
 	available = xe_oa_circ_diff(stream, stream->oa_buffer.tail, stream->oa_buffer.head);
 	stream->pollin = available >= stream->wait_num_reports * report_size;
-- 
2.51.0