drm/i915/perf: Clear out entire reports after reading if not power of 2 size

Clearing out report id and timestamp as means to detect unlanded reports
only works if report size is power of 2. That is, only when report size is
a sub-multiple of the OA buffer size can we be certain that reports will
land at the same place each time in the OA buffer (after rewind). If report
size is not a power of 2, we need to zero out the entire report to be able
to detect unlanded reports reliably.

v2: Add Fixes tag (Umesh)

Fixes: 1cc064dc ("drm/i915/perf: Add support for OA media units")
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230523204042.4180641-1-ashutosh.dixit@intel.com

drivers/gpu/drm/i915/i915_perf.c

@@ -877,12 +877,17 @@ static int gen8_append_oa_reports(struct i915_perf_stream *stream,
 			stream->oa_buffer.last_ctx_id = ctx_id;
 		}
 
-		/*
-		 * Clear out the report id and timestamp as a means to detect unlanded
-		 * reports.
-		 */
-		oa_report_id_clear(stream, report32);
-		oa_timestamp_clear(stream, report32);
+		if (is_power_of_2(report_size)) {
+			/*
+			 * Clear out the report id and timestamp as a means
+			 * to detect unlanded reports.
+			 */
+			oa_report_id_clear(stream, report32);
+			oa_timestamp_clear(stream, report32);
+		} else {
+			/* Zero out the entire report */
+			memset(report32, 0, report_size);
+		}
 	}
 
 	if (start_offset != *offset) {