• Chris Wilson's avatar
    drm/i915/execlists: Preempt-to-busy · 22b7a426
    Chris Wilson authored
    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: default avatarChris Wilson <chris@chris-wilson.co.uk>
    Reviewed-by: default avatarMika Kuoppala <mika.kuoppala@linux.intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20190620142052.19311-1-chris@chris-wilson.co.uk
    22b7a426
intel_engine.h 15.8 KB