drm/i915/gt: Limit frequency drop to RPe on parking

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 <[email protected]>
Tested-by: Edward Baker <[email protected]>
Fixes: 043cd2d ("drm/i915/gt: Leave rps->cur_freq on unpark")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Edward Baker <[email protected]>
Cc: Andi Shyti <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: <[email protected]> # v5.8+
Reviewed-by: Rodrigo Vivi <[email protected]>
Reviewed-by: Andi Shyti <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit f7ed83c)
Signed-off-by: Rodrigo Vivi <[email protected]>

drivers/gpu/drm/i915/gt/intel_rps.c

@@ -883,6 +883,10 @@ 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);
 }