drm/i915: Micro-optimise hotpath through intel_ring_begin()

Typically, there is space available within the ring and if not we have
to wait (by definition a slow path). Rearrange the code to reduce the
number of branches and stack size for the hotpath, accomodating a slight
growth for the wait.

v2: Fix the new assert that packets are not larger than the actual ring.
v3: Make the parameters unsigned as well to make usage.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170504130846.4807-3-chris@chris-wilson.co.uk

1 files changed, 36 insertions, 31 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index b308e73fcfae..acd1da9b62a3 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1656,7 +1656,8 @@ static int ring_request_alloc(struct drm_i915_gem_request *request)
         return 0;
 }
 
-static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
+static noinline int wait_for_space(struct drm_i915_gem_request *req,
+                                   unsigned int bytes)
 {
         struct intel_ring *ring = req->ring;
         struct drm_i915_gem_request *target;
@@ -1701,52 +1702,56 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
         return 0;
 }
 
-u32 *intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
+u32 *intel_ring_begin(struct drm_i915_gem_request *req,
+                      unsigned int num_dwords)
 {
         struct intel_ring *ring = req->ring;
-        int remain_actual = ring->size - ring->emit;
-        int remain_usable = ring->effective_size - ring->emit;
-        int bytes = num_dwords * sizeof(u32);
-        int total_bytes, wait_bytes;
-        bool need_wrap = false;
+        const unsigned int remain_usable = ring->effective_size - ring->emit;
+        const unsigned int bytes = num_dwords * sizeof(u32);
+        unsigned int need_wrap = 0;
+        unsigned int total_bytes;
         u32 *cs;
 
         total_bytes = bytes + req->reserved_space;
+        GEM_BUG_ON(total_bytes > ring->effective_size);
 
-        if (unlikely(bytes > remain_usable)) {
-                /*
-                 * Not enough space for the basic request. So need to flush
-                 * out the remainder and then wait for base + reserved.
-                 */
-                wait_bytes = remain_actual + total_bytes;
-                need_wrap = true;
-        } else if (unlikely(total_bytes > remain_usable)) {
-                /*
-                 * The base request will fit but the reserved space
-                 * falls off the end. So we don't need an immediate wrap
-                 * and only need to effectively wait for the reserved
-                 * size space from the start of ringbuffer.
-                 */
-                wait_bytes = remain_actual + req->reserved_space;
-        } else {
-                /* No wrapping required, just waiting. */
-                wait_bytes = total_bytes;
+        if (unlikely(total_bytes > remain_usable)) {
+                const int remain_actual = ring->size - ring->emit;
+
+                if (bytes > remain_usable) {
+                        /*
+                         * Not enough space for the basic request. So need to
+                         * flush out the remainder and then wait for
+                         * base + reserved.
+                         */
+                        total_bytes += remain_actual;
+                        need_wrap = remain_actual | 1;
+                } else  {
+                        /*
+                         * The base request will fit but the reserved space
+                         * falls off the end. So we don't need an immediate
+                         * wrap and only need to effectively wait for the
+                         * reserved size from the start of ringbuffer.
+                         */
+                        total_bytes = req->reserved_space + remain_actual;
+                }
         }
 
-        if (wait_bytes > ring->space) {
-                int ret = wait_for_space(req, wait_bytes);
+        if (unlikely(total_bytes > ring->space)) {
+                int ret = wait_for_space(req, total_bytes);
                 if (unlikely(ret))
                         return ERR_PTR(ret);
         }
 
         if (unlikely(need_wrap)) {
-                GEM_BUG_ON(remain_actual > ring->space);
-                GEM_BUG_ON(ring->emit + remain_actual > ring->size);
+                need_wrap &= ~1;
+                GEM_BUG_ON(need_wrap > ring->space);
+                GEM_BUG_ON(ring->emit + need_wrap > ring->size);
 
                 /* Fill the tail with MI_NOOP */
-                memset(ring->vaddr + ring->emit, 0, remain_actual);
+                memset(ring->vaddr + ring->emit, 0, need_wrap);
                 ring->emit = 0;
-                ring->space -= remain_actual;
+                ring->space -= need_wrap;
         }
 
         GEM_BUG_ON(ring->emit > ring->size - bytes);