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

1 files changed, 13 insertions, 11 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 3ce1c87dec46..46f2696bf886 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 @@ 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;
         }
 
@@ -1744,11 +1746,11 @@ u32 *intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
         }
 
         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;
 }