[PATCH 39/92] drm/i915: Add background commentary to "waitboosting"
chris at chris-wilson.co.uk
Thu May 26 10:11:45 UTC 2016
Describe the intent of boosting the GPU frequency to maximum before
waiting on the GPU.
RPS waitboosting was introduced with
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date: Wed Sep 25 17:34:56 2013 +0100
drm/i915: Boost RPS frequency for CPU stalls
but lacked a concise comment in the code to explain itself.
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
drivers/gpu/drm/i915/i915_gem.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 18a677e6e5b7..f5acb6ebb5ff 100644
@@ -1237,6 +1237,21 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
+ /* This client is about to stall waiting for the GPU. In many cases
+ * this is undesirable and limits the throughput of the system, as
+ * many clients cannot continue processing user input/output whilst
+ * blocked. RPS autotuning may take tens of milliseconds to respond
+ * to the GPU load and thus incurs additional latency for the client.
+ * We can circumvent that by promoting the GPU frequency to maximum
+ * before we wait. This makes the GPU throttle up much more quickly
+ * (good for benchmarks and user experience, e.g. window animations),
+ * but at a cost of spending more power processing the workload
+ * (bad for battery). Not all clients even want their results
+ * immediately and for them we should just let the GPU select its own
+ * frequency to maximise efficiency. To prevent a single client from
+ * forcing the clocks too high for the whole system, we only allow
+ * each client to waitboost once in a busy period.
if (INTEL_INFO(req->i915)->gen >= 6)
gen6_rps_boost(req->i915, rps, req->emitted_jiffies);
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