We only have to prevent the RING_TAIL from catching the RING_HEAD
cacheline and do not need to enforce a whole cacheline separation, and in
the process we can remove one branch from the computation.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/intel_ringbuffer.c | 20 ++++++++++----------
drivers/gpu/drm/i915/intel_ringbuffer.h | 24 +++++++++++++-----------
2 files changed, 23 insertions(+), 21 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 1ec98851a621..48d4b5b24b21 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -39,12 +39,15 @@
*/
#define LEGACY_REQUEST_SIZE 200
-static int __intel_ring_space(int head, int tail, int size)
+static unsigned int __intel_ring_space(unsigned int head,
+ unsigned int tail,
+ unsigned int size)
{
- int space = head - tail;
- if (space <= 0)
- space += size;
- return space - I915_RING_FREE_SPACE;
+ /* "If the Ring Buffer Head Pointer and the Tail Pointer are on the
+ * same cacheline, the Head Pointer must not be greater than the Tail
+ * Pointer."
+ */
+ return (round_down(head, CACHELINE_BYTES) - tail - 8) & (size - 1);
}
void intel_ring_update_space(struct intel_ring *ring)
@@ -1618,12 +1621,9 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
GEM_BUG_ON(!req->reserved_space);
list_for_each_entry(target, &ring->request_list, ring_link) {
- unsigned space;
-
/* Would completion of this request free enough space? */
- space = __intel_ring_space(target->postfix, ring->emit,
- ring->size);
- if (space >= bytes)
+ if (bytes <= __intel_ring_space(target->postfix,
+ ring->emit, ring->size))
break;
}
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 31998ea46fb8..e28ffc98c520 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -17,17 +17,6 @@
#define CACHELINE_BYTES 64
#define CACHELINE_DWORDS (CACHELINE_BYTES / sizeof(uint32_t))
-/*
- * Gen2 BSpec "1. Programming Environment" / 1.4.4.6 "Ring Buffer Use"
- * Gen3 BSpec "vol1c Memory Interface Functions" / 2.3.4.5 "Ring Buffer Use"
- * Gen4+ BSpec "vol1c Memory Interface and Command Stream" / 5.3.4.5 "Ring Buffer Use"
- *
- * "If the Ring Buffer Head Pointer and the Tail Pointer are on the same
- * cacheline, the Head Pointer must not be greater than the Tail
- * Pointer."
- */
-#define I915_RING_FREE_SPACE 64
-
struct intel_hw_status_page {
struct i915_vma *vma;
u32 *page_addr;
@@ -545,6 +534,19 @@ assert_ring_tail_valid(const struct intel_ring *ring, unsigned int tail)
*/
GEM_BUG_ON(!IS_ALIGNED(tail, 8));
GEM_BUG_ON(tail >= ring->size);
+
+ /* "Ring Buffer Use"
+ * Gen2 BSpec "1. Programming Environment" / 1.4.4.6
+ * Gen3 BSpec "1c Memory Interface Functions" / 2.3.4.5
+ * Gen4+ BSpec "1c Memory Interface and Command Stream" / 5.3.4.5
+ * "If the Ring Buffer Head Pointer and the Tail Pointer are on the
+ * same cacheline, the Head Pointer must not be greater than the Tail
+ * Pointer."
+ */
+#define cacheline(a) round_down(a, CACHELINE_BYTES)
+ GEM_BUG_ON(cacheline(tail) == cacheline(ring->head) &&
+ tail < ring->head);
+#undef cacheline
}
static inline unsigned int
--
2.11.0
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