1 files changed, 52 insertions, 0 deletions
diff --git a/drivers/gpu/drm/i915/intel_guc_fwif.h b/drivers/gpu/drm/i915/intel_guc_fwif.h
index 83143e8a0730..d73673f5d30c 100644
--- a/drivers/gpu/drm/i915/intel_guc_fwif.h
+++ b/drivers/gpu/drm/i915/intel_guc_fwif.h
@@ -327,6 +327,58 @@ struct guc_stage_desc {
 	u64 desc_private;
 } __packed;
 
+/**
+ * DOC: CTB based communication
+ *
+ * The CTB (command transport buffer) communication between Host and GuC
+ * is based on u32 data stream written to the shared buffer. One buffer can
+ * be used to transmit data only in one direction (one-directional channel).
+ *
+ * Current status of the each buffer is stored in the buffer descriptor.
+ * Buffer descriptor holds tail and head fields that represents active data
+ * stream. The tail field is updated by the data producer (sender), and head
+ * field is updated by the data consumer (receiver)::
+ *
+ *      +------------+
+ *      | DESCRIPTOR |          +=================+============+========+
+ *      +============+          |                 | MESSAGE(s) |        |
+ *      | address    |--------->+=================+============+========+
+ *      +------------+
+ *      | head       |          ^-----head--------^
+ *      +------------+
+ *      | tail       |          ^---------tail-----------------^
+ *      +------------+
+ *      | size       |          ^---------------size--------------------^
+ *      +------------+
+ *
+ * Each message in data stream starts with the single u32 treated as a header,
+ * followed by optional set of u32 data that makes message specific payload::
+ *
+ *      +------------+---------+---------+---------+
+ *      |         MESSAGE                          |
+ *      +------------+---------+---------+---------+
+ *      |   msg[0]   |   [1]   |   ...   |  [n-1]  |
+ *      +------------+---------+---------+---------+
+ *      |   MESSAGE  |       MESSAGE PAYLOAD       |
+ *      +   HEADER   +---------+---------+---------+
+ *      |            |    0    |   ...   |    n    |
+ *      +======+=====+=========+=========+=========+
+ *      | 31:16| code|         |         |         |
+ *      +------+-----+         |         |         |
+ *      |  15:5|flags|         |         |         |
+ *      +------+-----+         |         |         |
+ *      |   4:0|  len|         |         |         |
+ *      +------+-----+---------+---------+---------+
+ *
+ *                   ^-------------len-------------^
+ *
+ * The message header consists of:
+ *
+ * - **len**, indicates length of the message payload (in u32)
+ * - **code**, indicates message code
+ * - **flags**, holds various bits to control message handling
+ */
+
 /*
  * Describes single command transport buffer.
  * Used by both guc-master and clients.