[Intel-gfx] [PATCH v8 02/13] drm/i915/perf: add missing delay for OA muxes configuration

[Lionel Landwerlin]

This was dropped from the original patch series, we weren't sure
whether it was needed at the time. More recent tests show it's
definitely needed to have acurate performance data.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Fixes: 19f81df2859eb1 ("drm/i915/perf: Add OA unit support for Gen 8+")
Acked-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_perf.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
index 27842e7bcfed..545d3adaf341 100644
--- a/drivers/gpu/drm/i915/i915_perf.c
+++ b/drivers/gpu/drm/i915/i915_perf.c
@@ -1838,6 +1838,29 @@ static int gen8_enable_metric_set(struct i915_perf_stream *stream)
 
 	config_oa_regs(dev_priv, oa_config->mux_regs, oa_config->mux_regs_len);
 
+	/* It apparently takes a fairly long time for a new MUX
+	 * configuration to be be applied after these register writes.
+	 * This delay duration was derived empirically based on the
+	 * render_basic config but hopefully it covers the maximum
+	 * configuration latency.
+	 *
+	 * As a fallback, the checks in _append_oa_reports() to skip
+	 * invalid OA reports do also seem to work to discard reports
+	 * generated before this config has completed - albeit not
+	 * silently.
+	 *
+	 * Unfortunately this is essentially a magic number, since we
+	 * don't currently know of a reliable mechanism for predicting
+	 * how long the MUX config will take to apply and besides
+	 * seeing invalid reports we don't know of a reliable way to
+	 * explicitly check that the MUX config has landed.
+	 *
+	 * It's even possible we've miss characterized the underlying
+	 * problem - it just seems like the simplest explanation why
+	 * a delay at this location would mitigate any invalid reports.
+	 */
+	usleep_range(15000, 20000);
+
 	config_oa_regs(dev_priv, oa_config->b_counter_regs,
 		       oa_config->b_counter_regs_len);
 
-- 
2.22.0