Describe the intent of boosting the GPU frequency to maximum before
waiting on the GPU.
RPS waitboosting was introduced with
commit b29c19b645287f7062e17d70fa4e9781a01a5d88
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/i915_gem.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index ece7e2908fa3..40a7ade4eafe 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1248,6 +1248,22 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
}
trace_i915_gem_request_wait_begin(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
+ * asleep. 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 promoting the GPU frequency to maximum
+ * before we wait. This makes GPU throttle up much more quickly
+ * (good for benchmarks), 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);
--
2.6.2
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