drm/i915/gt: Defer enabling the breadcrumb interrupt to after submission
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: e2300560 ("drm/i915/gt: Hold context/request reference while breadcrumbs are active") Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20201123113717.20500-1-chris@chris-wilson.co.uk
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