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drm/i915/execlists: Preemption!
When we write to ELSP, it triggers a context preemption at the earliest arbitration point (3DPRIMITIVE, some PIPECONTROLs, a few other operations and the explicit MI_ARB_CHECK). If this is to the same context, it triggers a LITE_RESTORE where the RING_TAIL is merely updated (used currently to chain requests from the same context together, avoiding bubbles). However, if it is to a different context, a full context-switch is performed and it will start to execute the new context saving the image of the old for later execution. Previously we avoided preemption by only submitting a new context when the old was idle. But now we wish embrace it, and if the new request has a higher priority than the currently executing request, we write to the ELSP regardless, thus triggering preemption, but we tell the GPU to switch to our special preemption context (not the target). In the context-switch interrupt handler, we know that the previous contexts have finished execution and so can unwind all the incomplete requests and compute the new highest priority request to execute. It would be feasible to avoid the switch-to-idle intermediate by programming the ELSP with the target context. The difficulty is in tracking which request that should be whilst maintaining the dependency change, the error comes in with coalesced requests. As we only track the most recent request and its priority, we may run into the issue of being tricked in preempting a high priority request that was followed by a low priority request from the same context (e.g. for PI); worse still that earlier request may be our own dependency and the order then broken by preemption. By injecting the switch-to-idle and then recomputing the priority queue, we avoid the issue with tracking in-flight coalesced requests. Having tried the preempt-to-busy approach, and failed to find a way around the coalesced priority issue, Michal's original proposal to inject an idle context (based on handling GuC preemption) succeeds. The current heuristic for deciding when to preempt are only if the new request is of higher priority, and has the privileged priority of greater than 0. Note that the scheduler remains unfair! v2: Disable for gen8 (bdw/bsw) as we need additional w/a for GPGPU. Since, the feature is now conditional and not always available when we have a scheduler, make it known via the HAS_SCHEDULER GETPARAM (now a capability mask). v3: Stylistic tweaks. v4: Appease Joonas with a snippet of kerneldoc, only to fuel to fire of the preempt vs preempting debate. Suggested-by:Michal Winiarski <michal.winiarski@intel.com> Signed-off-by: