[Bug 99671] [v4.10 snb] weird seqno/request tracking - HEAD overtakes TAIL

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=99671

--- Comment #44 from Chris Wilson <chris at chris-wilson.co.uk> ---
Test to see if we ever write requests out-of-order:

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c
b/drivers/gpu/drm/i915/intel_ringbuffer.c
index be908e2a52ea..da610ce176a9 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -784,6 +784,16 @@ static void i9xx_submit_request(struct
drm_i915_gem_request *request)

        i915_gem_request_submit(request);

+       {
+               u32 head = I915_READ_HEAD(request->engine) & HEAD_ADDR;
+               u32 tail = I915_READ_TAIL(request->engine) & HEAD_ADDR;
+               int prev = __intel_ring_space(tail, head, request->ring->size);
+               int next = __intel_ring_space(request->tail, head,
request->ring->size);
+               WARN(head != tail && next <= prev,
+                    "Bacwards we go: head=%x, tail=%x, next=%x\n",
+                    head, tail, request->tail);
+       }
+
        GEM_BUG_ON(!IS_ALIGNED(request->tail, 8));
        I915_WRITE_TAIL(request->engine, request->tail);
 }

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