• Chris Wilson's avatar
    drm/i915/gt: Incrementally check for rewinding · e36ba817
    Chris Wilson authored
    In commit 5ba32c7b ("drm/i915/execlists: Always force a context
    reload when rewinding RING_TAIL"), we placed the check for rewinding a
    context on actually submitting the next request in that context. This
    was so that we only had to check once, and could do so with precision
    avoiding as many forced restores as possible. For example, to ensure
    that we can resubmit the same request a couple of times, we include a
    small wa_tail such that on the next submission, the ring->tail will
    appear to move forwards when resubmitting the same request. This is very
    common as it will happen for every lite-restore to fill the second port
    after a context switch.
    
    However, intel_ring_direction() is limited in precision to movements of
    upto half the ring size. The consequence being that if we tried to
    unwind many requests, we could exceed half the ring and flip the sense
    of the direction, so missing a force restore. As no request can be
    greater than half the ring (i.e. 2048 bytes in the smallest case), we
    can check for rollback incrementally. As we check against the tail that
    would be submitted, we do not lose any sensitivity and allow lite
    restores for the simple case. We still need to double check upon
    submitting the context, to allow for multiple preemptions and
    resubmissions.
    
    Fixes: 5ba32c7b ("drm/i915/execlists: Always force a context reload when rewinding RING_TAIL")
    Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
    Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
    Cc: <stable@vger.kernel.org> # v5.4+
    Reviewed-by: default avatarBruce Chang <yu.bruce.chang@intel.com>
    Reviewed-by: default avatarMika Kuoppala <mika.kuoppala@linux.intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20200609151723.12971-1-chris@chris-wilson.co.uk
    e36ba817
intel_ring.c 7.35 KB