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drm/i915: Add background commentary to "waitboosting"

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Describe the intent of boosting the GPU frequency to maximum before
waiting on the GPU.

RPS waitboosting was introduced with commit b29c19b645 ("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@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1467616119-4093-5-git-send-email-chris@chris-wilson.co.uk
This commit is contained in:
Chris Wilson
2016-07-04 08:08:35 +01:00
parent 0e6883b043
commit df4ba5099f
1 changed files with 15 additions and 0 deletions
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15
drivers/gpu/drm/i915/i915_gem.c
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@@ -1462,6 +1462,21 @@ 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
* 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);