[Intel-gfx] [PATCH] drm/i915/gt: Limit frequency drop to RPe on parking

[Chris Wilson]

We treat idling the GT (intel_rps_park) as a downclock event, and reduce
the frequency we intend to restart the GT with. Since the two workloads
are likely related (e.g. a compositor rendering every 16ms), we want to
carry the frequency and load information from across the idling.
However, we do also need to update the frequencies so that workloads
that run for less than 1ms are autotuned by RPS (otherwise we leave
compositors running at max clocks, draining excess power). Conversely,
if we try to run too slowly, the next workload has to run longer. Since
there is a hysteresis in the power graph, below a certain frequency
running a short workload for longer consumes more energy than running it
slightly higher for less time. The exact balance point is unknown
beforehand, but measurements with 30fps media playback indicate that RPe
is a better choice.

Reported-by: Edward Baker <edward.baker at intel.com>
Fixes: 043cd2d14ede ("drm/i915/gt: Leave rps->cur_freq on unpark")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Edward Baker <edward.baker at intel.com>
Cc: Andi Shyti <andi.shyti at intel.com>
Cc: Lyude Paul <lyude at redhat.com>
Cc: <stable at vger.kernel.org> # v5.8+
---
 drivers/gpu/drm/i915/gt/intel_rps.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/i915/gt/intel_rps.c b/drivers/gpu/drm/i915/gt/intel_rps.c
index b13e7845d483..f74d5e09e176 100644
--- a/drivers/gpu/drm/i915/gt/intel_rps.c
+++ b/drivers/gpu/drm/i915/gt/intel_rps.c
@@ -907,6 +907,10 @@ void intel_rps_park(struct intel_rps *rps)
 		adj = -2;
 	rps->last_adj = adj;
 	rps->cur_freq = max_t(int, rps->cur_freq + adj, rps->min_freq);
+	if (rps->cur_freq < rps->efficient_freq) {
+		rps->cur_freq = rps->efficient_freq;
+		rps->last_adj = 0;
+	}
 
 	GT_TRACE(rps_to_gt(rps), "park:%x\n", rps->cur_freq);
 }
-- 
2.20.1