drm/i915/gt: Incrementally check for rewinding

In commit 5ba32c7be81e ("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: 5ba32c7be81e ("drm/i915/execlists: Always force a context reload when rewinding RING_TAIL")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: <[email protected]> # v5.4+
Reviewed-by: Bruce Chang <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
index e5141a897786..0a05301e00fb 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
@@ -646,7 +646,7 @@ static int engine_setup_common(struct intel_engine_cs *engine)
 struct measure_breadcrumb {
 	struct i915_request rq;
 	struct intel_ring ring;
-	u32 cs[1024];
+	u32 cs[2048];
 };
 
 static int measure_breadcrumb_dw(struct intel_context *ce)
@@ -667,6 +667,8 @@ static int measure_breadcrumb_dw(struct intel_context *ce)
 
 	frame->ring.vaddr = frame->cs;
 	frame->ring.size = sizeof(frame->cs);
+	frame->ring.wrap =
+		BITS_PER_TYPE(frame->ring.size) - ilog2(frame->ring.size);
 	frame->ring.effective_size = frame->ring.size;
 	intel_ring_update_space(&frame->ring);
 	frame->rq.ring = &frame->ring;