On retiring the request, we should not re-use these elements in the ring
(at least not until we fill the ringbuffer and knowingly reuse the space).
Leave behind some poison to (hopefully) trap ourselves if we make a
mistake.

Suggested-by: Mika Kuoppala <mika.kuopp...@linux.intel.com>
Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuopp...@linux.intel.com>
---
 drivers/gpu/drm/i915/i915_request.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c 
b/drivers/gpu/drm/i915/i915_request.c
index 0ecc2cf64216..9ee7bf0200b0 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -203,6 +203,19 @@ static void free_capture_list(struct i915_request *request)
        }
 }
 
+static void __i915_request_fill(struct i915_request *rq, u8 val)
+{
+       void *vaddr = rq->ring->vaddr;
+       u32 head;
+
+       head = rq->infix;
+       if (rq->postfix < head) {
+               memset(vaddr + head, val, rq->ring->size - head);
+               head = 0;
+       }
+       memset(vaddr + head, val, rq->postfix - head);
+}
+
 static void remove_from_engine(struct i915_request *rq)
 {
        struct intel_engine_cs *engine, *locked;
@@ -247,6 +260,9 @@ bool i915_request_retire(struct i915_request *rq)
         */
        GEM_BUG_ON(!list_is_first(&rq->link,
                                  &i915_request_timeline(rq)->requests));
+       if (IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM))
+               /* Poison before we release our space in the ring */
+               __i915_request_fill(rq, POISON_FREE);
        rq->ring->head = rq->postfix;
 
        /*
@@ -1175,9 +1191,6 @@ i915_request_await_object(struct i915_request *to,
 
 void i915_request_skip(struct i915_request *rq, int error)
 {
-       void *vaddr = rq->ring->vaddr;
-       u32 head;
-
        GEM_BUG_ON(!IS_ERR_VALUE((long)error));
        dma_fence_set_error(&rq->fence, error);
 
@@ -1189,12 +1202,7 @@ void i915_request_skip(struct i915_request *rq, int 
error)
         * context, clear out all the user operations leaving the
         * breadcrumb at the end (so we get the fence notifications).
         */
-       head = rq->infix;
-       if (rq->postfix < head) {
-               memset(vaddr + head, 0, rq->ring->size - head);
-               head = 0;
-       }
-       memset(vaddr + head, 0, rq->postfix - head);
+       __i915_request_fill(rq, 0);
        rq->infix = rq->postfix;
 }
 
-- 
2.25.0

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