drm/nouveau: add a kernel doc to introduce the GSP RPC

In order to explain the name clean-ups in GSP RPC routines, a kernel
doc to explain the memory layout and terms is required.

Add a kernel doc to introduce the GSP RPC.

Cc: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Zhi Wang <zhiw@nvidia.com>
[ Fix bullet list indentation; add SPDX-License-Identifier. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250124182958.2040494-2-zhiw@nvidia.com

Documentation/gpu/drivers.rst

@@ -10,6 +10,7 @@ GPU Driver Documentation
    imagination/index
    mcde
    meson
+   nouveau
    pl111
    tegra
    tve200

Documentation/gpu/nouveau.rst

+.. SPDX-License-Identifier: (GPL-2.0+ OR MIT)
+
+===============================
+ drm/nouveau NVIDIA GPU Driver
+===============================
+
+The drm/nouveau driver provides support for a wide range of NVIDIA GPUs,
+covering GeForce, Quadro, and Tesla series, from the NV04 architecture up
+to the latest Turing, Ampere, Ada families.
+
+NVKM: NVIDIA Kernel Manager
+===========================
+
+The NVKM component serves as the core abstraction layer within the nouveau
+driver, responsible for managing NVIDIA GPU hardware at the kernel level.
+NVKM provides a unified interface for handling various GPU  architectures.
+
+It enables resource management, power control, memory handling, and command
+submission required for the proper functioning of NVIDIA GPUs under the
+nouveau driver.
+
+NVKM plays a critical role in abstracting hardware complexities and
+providing a consistent API to upper layers of the driver stack.
+
+GSP Support
+------------------------
+
+.. kernel-doc:: drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c
+   :doc: GSP message queue element

drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c

@@ -63,6 +63,51 @@ extern struct dentry *nouveau_debugfs_root;
 #define GSP_MSG_MIN_SIZE GSP_PAGE_SIZE
 #define GSP_MSG_MAX_SIZE GSP_PAGE_MIN_SIZE * 16
 
+/**
+ * DOC: GSP message queue element
+ *
+ * https://github.com/NVIDIA/open-gpu-kernel-modules/blob/535/src/nvidia/inc/kernel/gpu/gsp/message_queue_priv.h
+ *
+ * The GSP command queue and status queue are message queues for the
+ * communication between software and GSP. The software submits the GSP
+ * RPC via the GSP command queue, GSP writes the status of the submitted
+ * RPC in the status queue.
+ *
+ * A GSP message queue element consists of three parts:
+ *
+ * - message element header (struct r535_gsp_msg), which mostly maintains
+ *   the metadata for queuing the element.
+ *
+ * - RPC message header (struct nvfw_gsp_rpc), which maintains the info
+ *   of the RPC. E.g., the RPC function number.
+ *
+ * - The payload, where the RPC message stays. E.g. the params of a
+ *   specific RPC function. Some RPC functions also have their headers
+ *   in the payload. E.g. rm_alloc, rm_control.
+ *
+ * The memory layout of a GSP message element can be illustrated below::
+ *
+ *    +------------------------+
+ *    | Message Element Header |
+ *    |    (r535_gsp_msg)      |
+ *    |                        |
+ *    | (r535_gsp_msg.data)    |
+ *    |          |             |
+ *    |----------V-------------|
+ *    |    GSP RPC Header      |
+ *    |    (nvfw_gsp_rpc)      |
+ *    |                        |
+ *    | (nvfw_gsp_rpc.data)    |
+ *    |          |             |
+ *    |----------V-------------|
+ *    |       Payload          |
+ *    |                        |
+ *    |   header(optional)     |
+ *    |        params          |
+ *    +------------------------+
+ *
+ */
+
 struct r535_gsp_msg {
 	u8 auth_tag_buffer[16];
 	u8 aad_buffer[16];