drm/i915: Move the modulus for ring emission to the register write

Space reservation is already safe with respect to the ring->size
modulus, but hardware only expects to see values in the range
0...ring->size-1 (inclusive) and so requires the modulus to prevent us
writing the value ring->size instead of 0. As this is only required for
the register itself, we can defer the modulus to the register update and
not perform it after every command packet. We keep the
intel_ring_advance() around in the code to provide demarcation for the
end-of-packet (which then can be compared against intel_ring_begin() as
the number of dwords emitted must match the reserved space).

v2: Assert that the ring size is a power-of-two to match assumptions in
the code. Simplify the comment before writing the tail value to explain
why the modulus is necessary.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-13-git-send-email-chris@chris-wilson.co.uk

1 files changed, 6 insertions, 2 deletions

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 3142085..a039353 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1718,7 +1718,8 @@ static void i9xx_submit_request(struct drm_i915_gem_request *request)
 {
 	struct drm_i915_private *dev_priv = request->i915;
 
-	I915_WRITE_TAIL(request->engine, request->tail);
+	I915_WRITE_TAIL(request->engine,
+			intel_ring_offset(request->ring, request->tail));
 }
 
 static void
@@ -2081,6 +2082,8 @@ intel_engine_create_ring(struct intel_engine_cs *engine, int size)
 	struct intel_ring *ring;
 	int ret;
 
+	GEM_BUG_ON(!is_power_of_2(size));
+
 	ring = kzalloc(sizeof(*ring), GFP_KERNEL);
 	if (ring == NULL) {
 		DRM_DEBUG_DRIVER("Failed to allocate ringbuffer %s\n",
@@ -2505,7 +2508,8 @@ static void gen6_bsd_submit_request(struct drm_i915_gem_request *request)
 		DRM_ERROR("timed out waiting for the BSD ring to wake up\n");
 
 	/* Now that the ring is fully powered up, update the tail */
-	I915_WRITE_FW(RING_TAIL(request->engine->mmio_base), request->tail);
+	I915_WRITE_FW(RING_TAIL(request->engine->mmio_base),
+		      intel_ring_offset(request->ring, request->tail));
 	POSTING_READ_FW(RING_TAIL(request->engine->mmio_base));
 
 	/* Let the ring send IDLE messages to the GT again,