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When using a global seqno, we required a precise stop-the-workd event to handle preemption and unwind the global seqno counter. To accomplish this, we would preempt to a special out-of-band context and wait for the machine to report that it was idle. Given an idle machine, we could very precisely see which requests had completed and which we needed to feed back into the run queue. However, now that we have scrapped the global seqno, we no longer need to precisely unwind the global counter and only track requests by their per-context seqno. This allows us to loosely unwind inflight requests while scheduling a preemption, with the enormous caveat that the requests we put back on the run queue are still _inflight_ (until the preemption request is complete). This makes request tracking much more messy, as at any point then we can see a completed request that we believe is not currently scheduled for execution. We also have to be careful not to rewind RING_TAIL past RING_HEAD on preempting to the running context, and for this we use a semaphore to prevent completion of the request before continuing. To accomplish this feat, we change how we track requests scheduled to the HW. Instead of appending our requests onto a single list as we submit, we track each submission to ELSP as its own block. Then upon receiving the CS preemption event, we promote the pending block to the inflight block (discarding what was previously being tracked). As normal CS completion events arrive, we then remove stale entries from the inflight tracker. v2: Be a tinge paranoid and ensure we flush the write into the HWS page for the GPU semaphore to pick in a timely fashion. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20190620142052.19311-1-chris@chris-wilson.co.uk |
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