drm/i915: Avoid the branch in computing intel_ring_space()
Exploit the power-of-two ring size to compute the space across the
wraparound using a mask rather than a if. Convert to unsigned integers
so the operation is well defined.
References: https://bugs.freedesktop.org/show_bug.cgi?id=99671
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170504130846.4807-1-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 3ce1c87..46f2696 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -39,12 +39,17 @@
*/
#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."
+ */
+ GEM_BUG_ON(!is_power_of_2(size));
+ return (head - tail - CACHELINE_BYTES) & (size - 1);
}
void intel_ring_update_space(struct intel_ring *ring)
@@ -1670,12 +1675,9 @@
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;
}
@@ -1744,11 +1746,11 @@
}
GEM_BUG_ON(ring->emit > ring->size - bytes);
+ GEM_BUG_ON(ring->space < bytes);
cs = ring->vaddr + ring->emit;
GEM_DEBUG_EXEC(memset(cs, POISON_INUSE, bytes));
ring->emit += bytes;
ring->space -= bytes;
- GEM_BUG_ON(ring->space < 0);
return cs;
}
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 600713b..650ab88 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;
@@ -145,9 +134,9 @@
u32 tail;
u32 emit;
- int space;
- int size;
- int effective_size;
+ u32 space;
+ u32 size;
+ u32 effective_size;
};
struct i915_gem_context;
@@ -548,6 +537,25 @@
*/
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."
+ *
+ * We use ring->head as the last known location of the actual RING_HEAD,
+ * it may have advanced but in the worst case it is equally the same
+ * as ring->head and so we should never program RING_TAIL to advance
+ * into the same cacheline as ring->head.
+ */
+#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