drm/i915: Share the computation of ring size for RING_CTL register

Since both legacy and execlists want to populate the RING_CTL register,
share the computation of the right bits for the ring->size. We can then
stop masking errors and explicitly forbid them during creation!

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/20161004201132.21801-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_reg.h

@@ -1605,6 +1605,7 @@ enum skl_disp_power_wells {
 #define RING_HEAD(base)		_MMIO((base)+0x34)
 #define RING_START(base)	_MMIO((base)+0x38)
 #define RING_CTL(base)		_MMIO((base)+0x3c)
+#define   RING_CTL_SIZE(size)	((size) - PAGE_SIZE) /* in bytes -> pages */
 #define RING_SYNC_0(base)	_MMIO((base)+0x40)
 #define RING_SYNC_1(base)	_MMIO((base)+0x44)
 #define RING_SYNC_2(base)	_MMIO((base)+0x48)

drivers/gpu/drm/i915/intel_lrc.c

@@ -1946,7 +1946,7 @@ populate_lr_context(struct i915_gem_context *ctx,
 		       RING_START(engine->mmio_base), 0);
 	ASSIGN_CTX_REG(reg_state, CTX_RING_BUFFER_CONTROL,
 		       RING_CTL(engine->mmio_base),
-		       ((ring->size - PAGE_SIZE) & RING_NR_PAGES) | RING_VALID);
+		       RING_CTL_SIZE(ring->size) | RING_VALID);
 	ASSIGN_CTX_REG(reg_state, CTX_BB_HEAD_U,
 		       RING_BBADDR_UDW(engine->mmio_base), 0);
 	ASSIGN_CTX_REG(reg_state, CTX_BB_HEAD_L,

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -585,9 +585,7 @@ static int init_ring_common(struct intel_engine_cs *engine)
 	I915_WRITE_TAIL(engine, ring->tail);
 	(void)I915_READ_TAIL(engine);
 
-	I915_WRITE_CTL(engine,
-			((ring->size - PAGE_SIZE) & RING_NR_PAGES)
-			| RING_VALID);
+	I915_WRITE_CTL(engine, RING_CTL_SIZE(ring->size) | RING_VALID);
 
 	/* If the head is still not zero, the ring is dead */
 	if (intel_wait_for_register_fw(dev_priv, RING_CTL(engine->mmio_base),
@@ -1951,6 +1949,7 @@ intel_engine_create_ring(struct intel_engine_cs *engine, int size)
 	struct i915_vma *vma;
 
 	GEM_BUG_ON(!is_power_of_2(size));
+	GEM_BUG_ON(RING_CTL_SIZE(size) & ~RING_NR_PAGES);
 
 	ring = kzalloc(sizeof(*ring), GFP_KERNEL);
 	if (!ring)