drm/i915: Reserve space improvements

An earlier patch was added to reserve space in the ring buffer for the
commands issued during 'add_request()'. The initial version was
pessimistic in the way it handled buffer wrapping and would cause
premature wraps and thus waste ring space.

This patch updates the code to better handle the wrap case. It no
longer enforces that the space being asked for and the reserved space
are a single contiguous block. Instead, it allows the reserve to be on
the far end of a wrap operation. It still guarantees that the space is
available so when the wrap occurs, no wait will happen. Thus the wrap
cannot fail which is the whole point of the exercise.

Also fixed a merge failure with some comments from the original patch.

v2: Incorporated suggestion by David Gordon to move the wrap code
inside the prepare function and thus allow a single combined
wait_for_space() call rather than doing one before the wrap and
another after. This also makes the prepare code much simpler and
easier to follow.

v3: Fix for 'effective_size' vs 'size' during ring buffer remainder
calculations (spotted by Tomas Elf).

For: VIZ-5115
CC: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Tomas Elf <tomas.elf@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 36 insertions, 37 deletions

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 8cac4cab1666..22e9f85f40e4 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -661,12 +661,12 @@ static int logical_ring_wait_for_space(struct drm_i915_gem_request *req,
 	unsigned space;
 	int ret;
 
-	/* The whole point of reserving space is to not wait! */
-	WARN_ON(ringbuf->reserved_in_use);
-
 	if (intel_ring_space(ringbuf) >= bytes)
 		return 0;
 
+	/* The whole point of reserving space is to not wait! */
+	WARN_ON(ringbuf->reserved_in_use);
+
 	list_for_each_entry(target, &ring->request_list, list) {
 		/*
 		 * The request queue is per-engine, so can contain requests
@@ -716,22 +716,11 @@ intel_logical_ring_advance_and_submit(struct drm_i915_gem_request *request)
 	execlists_context_queue(request);
 }
 
-static int logical_ring_wrap_buffer(struct drm_i915_gem_request *req)
+static void __wrap_ring_buffer(struct intel_ringbuffer *ringbuf)
 {
-	struct intel_ringbuffer *ringbuf = req->ringbuf;
 	uint32_t __iomem *virt;
 	int rem = ringbuf->size - ringbuf->tail;
 
-	/* Can't wrap if space has already been reserved! */
-	WARN_ON(ringbuf->reserved_in_use);
-
-	if (ringbuf->space < rem) {
-		int ret = logical_ring_wait_for_space(req, rem);
-
-		if (ret)
-			return ret;
-	}
-
 	virt = ringbuf->virtual_start + ringbuf->tail;
 	rem /= 4;
 	while (rem--)
@@ -739,40 +728,50 @@ static int logical_ring_wrap_buffer(struct drm_i915_gem_request *req)
 
 	ringbuf->tail = 0;
 	intel_ring_update_space(ringbuf);
-
-	return 0;
 }
 
 static int logical_ring_prepare(struct drm_i915_gem_request *req, int bytes)
 {
 	struct intel_ringbuffer *ringbuf = req->ringbuf;
-	int ret;
-
-	/*
-	 * Add on the reserved size to the request to make sure that after
-	 * the intended commands have been emitted, there is guaranteed to
-	 * still be enough free space to send them to the hardware.
-	 */
-	if (!ringbuf->reserved_in_use)
-		bytes += ringbuf->reserved_size;
-
-	if (unlikely(ringbuf->tail + bytes > ringbuf->effective_size)) {
-		ret = logical_ring_wrap_buffer(req);
-		if (unlikely(ret))
-			return ret;
+	int remain_usable = ringbuf->effective_size - ringbuf->tail;
+	int remain_actual = ringbuf->size - ringbuf->tail;
+	int ret, total_bytes, wait_bytes = 0;
+	bool need_wrap = false;
 
-		if(ringbuf->reserved_size) {
-			uint32_t size = ringbuf->reserved_size;
+	if (ringbuf->reserved_in_use)
+		total_bytes = bytes;
+	else
+		total_bytes = bytes + ringbuf->reserved_size;
 
-			intel_ring_reserved_space_cancel(ringbuf);
-			intel_ring_reserved_space_reserve(ringbuf, 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 only need to to wait for the
+			 * reserved size after flushing out the remainder.
+			 */
+			wait_bytes = remain_actual + ringbuf->reserved_size;
+			need_wrap = true;
+		} else if (total_bytes > ringbuf->space) {
+			/* No wrapping required, just waiting. */
+			wait_bytes = total_bytes;
 		}
 	}
 
-	if (unlikely(ringbuf->space < bytes)) {
-		ret = logical_ring_wait_for_space(req, bytes);
+	if (wait_bytes) {
+		ret = logical_ring_wait_for_space(req, wait_bytes);
 		if (unlikely(ret))
 			return ret;
+
+		if (need_wrap)
+			__wrap_ring_buffer(ringbuf);
 	}
 
 	return 0;