[Intel-gfx] [PATCH 3/7] drm/i915/gt: Defer enabling the breadcrumb interrupt to after submission

[Chris Wilson]

Quoting Chris Wilson (2020-08-07 09:32:52)
> Move the register slow register write and readback from out of the
> critical path for execlists submission and delay it until the following
> worker, shaving off around 200us. Note that the same signal_irq_work() is
> allowed to run concurrently on each CPU (but it will only be queued once,
> once running though it can be requeued and reexecuted) so we have to
> remember to lock the global interactions as we cannot rely on the
> signal_irq_work() itself providing the serialisation (in constrast to a
> tasklet).
> 
> By pushing the arm/disarm into the central signaling worker we can close
> the race for disarming the interrupt (and dropping its associated
> GT wakeref) on parking the engine. If we loose the race, that GT wakeref
> may be held indefinitely, preventing the machine from sleeping while
> the GPU is ostensibly idle.
> 
> v2: Move the self-arming parking of the signal_irq_work to a flush of
> the irq-work from intel_breadcrumbs_park().
 
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2271
> Fixes: dfeba1ae34c8 ("drm/i915/gt: Hold context/request reference while breadcrumbs are active")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris