drm/i915: Cleanup some of the CSB handling

I think this patch is a worthwhile cleanup even if it might look only marginally
useful. It gets more useful in upcoming patches and for handling of future GEN
platforms.

The only non-mechanical part of this is the removal of the extra & operation on
the ring->next_context_status_buffer. This is safe because right above this, we
already did a modulus operation.

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452018609-10142-2-git-send-email-benjamin.widawsky@intel.com
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.h

@@ -25,8 +25,6 @@
 #define _INTEL_LRC_H_
 
 #define GEN8_LR_CONTEXT_ALIGN 4096
-#define GEN8_CSB_ENTRIES 6
-#define GEN8_CSB_PTR_MASK 0x07
 
 /* Execlists regs */
 #define RING_ELSP(ring)				_MMIO((ring)->mmio_base + 0x230)
@@ -40,6 +38,22 @@
 #define RING_CONTEXT_STATUS_BUF_HI(ring, i)	_MMIO((ring)->mmio_base + 0x370 + (i) * 8 + 4)
 #define RING_CONTEXT_STATUS_PTR(ring)		_MMIO((ring)->mmio_base + 0x3a0)
 
+/* The docs specify that the write pointer wraps around after 5h, "After status
+ * is written out to the last available status QW at offset 5h, this pointer
+ * wraps to 0."
+ *
+ * Therefore, one must infer than even though there are 3 bits available, 6 and
+ * 7 appear to be * reserved.
+ */
+#define GEN8_CSB_ENTRIES 6
+#define GEN8_CSB_PTR_MASK 0x7
+#define GEN8_CSB_READ_PTR_MASK (GEN8_CSB_PTR_MASK << 8)
+#define GEN8_CSB_WRITE_PTR_MASK (GEN8_CSB_PTR_MASK << 0)
+#define GEN8_CSB_WRITE_PTR(csb_status) \
+	(((csb_status) & GEN8_CSB_WRITE_PTR_MASK) >> 0)
+#define GEN8_CSB_READ_PTR(csb_status) \
+	(((csb_status) & GEN8_CSB_READ_PTR_MASK) >> 8)
+
 /* Logical Rings */
 int intel_logical_ring_alloc_request_extras(struct drm_i915_gem_request *request);
 int intel_logical_ring_reserve_space(struct drm_i915_gem_request *request);