drm/i915/guc: Increase size of CTB buffers

With the introduction of non-blocking CTBs more than one CTB can be in
flight at a time. Increasing the size of the CTBs should reduce how
often software hits the case where no space is available in the CTB
buffer.

Cc: John Harrison <john.c.harrison@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708162055.129996-4-matthew.brost@intel.com

drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c

@@ -58,11 +58,16 @@ static inline struct drm_device *ct_to_drm(struct intel_guc_ct *ct)
  *      +--------+-----------------------------------------------+------+
  *
  * Size of each `CT Buffer`_ must be multiple of 4K.
- * As we don't expect too many messages, for now use minimum sizes.
+ * We don't expect too many messages in flight at any time, unless we are
+ * using the GuC submission. In that case each request requires a minimum
+ * 2 dwords which gives us a maximum 256 queue'd requests. Hopefully this
+ * enough space to avoid backpressure on the driver. We increase the size
+ * of the receive buffer (relative to the send) to ensure a G2H response
+ * CTB has a landing spot.
  */
 #define CTB_DESC_SIZE		ALIGN(sizeof(struct guc_ct_buffer_desc), SZ_2K)
 #define CTB_H2G_BUFFER_SIZE	(SZ_4K)
-#define CTB_G2H_BUFFER_SIZE	(SZ_4K)
+#define CTB_G2H_BUFFER_SIZE	(4 * CTB_H2G_BUFFER_SIZE)
 
 struct ct_request {
 	struct list_head link;
@@ -643,7 +648,7 @@ static int ct_read(struct intel_guc_ct *ct, struct ct_incoming_msg **msg)
 	/* beware of buffer wrap case */
 	if (unlikely(available < 0))
 		available += size;
-	CT_DEBUG(ct, "available %d (%u:%u)\n", available, head, tail);
+	CT_DEBUG(ct, "available %d (%u:%u:%u)\n", available, head, tail, size);
 	GEM_BUG_ON(available < 0);
 
 	header = cmds[head];