drm/i915: Signal first fence from irq handler if complete
As execlists and other non-semaphore multi-engine devices coordinate between engines using interrupts, we can shave off a few 10s of microsecond of scheduling latency by doing the fence signaling from the interrupt as opposed to a RT kthread. (Realistically the delay adds about 1% to an individual cross-engine workload.) We only signal the first fence in order to limit the amount of work we move into the interrupt handler. We also have to remember that our breadcrumbs may be unordered with respect to the interrupt and so we still require the waiter process to perform some heavyweight coherency fixups, as well as traversing the tree of waiters. v2: No need for early exit in irq handler - it breaks the flow between patches and prevents the tracepoint v3: Restore rcu hold across irq signaling of request Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com> Reviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/20170227205850.2828-2-chris@chris-wilson.co.uk
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