drm/doc: document DRM_IOCTL_MODE_CREATE_DUMB

The main motivation is to repeat that dumb buffers should not be
abused for anything else than basic software rendering with KMS.
User-space devs are more likely to look at the IOCTL docs than to
actively search for the driver-oriented "Dumb Buffer Objects"
section.

v2: reference DRM_CAP_DUMB_BUFFER, DRM_CAP_DUMB_PREFERRED_DEPTH and
DRM_CAP_DUMB_PREFER_SHADOW (Pekka)

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230821131548.269204-1-contact@emersion.fr

Author: emersion

Documentation/gpu/drm-kms.rst

@@ -360,6 +360,8 @@ Format Functions Reference
 .. kernel-doc:: drivers/gpu/drm/drm_fourcc.c
    :export:
 
+.. _kms_dumb_buffer_objects:
+
 Dumb Buffer Objects
 ===================
 

include/uapi/drm/drm.h

@@ -1134,6 +1134,26 @@ extern "C" {
 #define DRM_IOCTL_MODE_PAGE_FLIP	DRM_IOWR(0xB0, struct drm_mode_crtc_page_flip)
 #define DRM_IOCTL_MODE_DIRTYFB		DRM_IOWR(0xB1, struct drm_mode_fb_dirty_cmd)
 
+/**
+ * DRM_IOCTL_MODE_CREATE_DUMB - Create a new dumb buffer object.
+ *
+ * KMS dumb buffers provide a very primitive way to allocate a buffer object
+ * suitable for scanout and map it for software rendering. KMS dumb buffers are
+ * not suitable for hardware-accelerated rendering nor video decoding. KMS dumb
+ * buffers are not suitable to be displayed on any other device than the KMS
+ * device where they were allocated from. Also see
+ * :ref:`kms_dumb_buffer_objects`.
+ *
+ * The IOCTL argument is a struct drm_mode_create_dumb.
+ *
+ * User-space is expected to create a KMS dumb buffer via this IOCTL, then add
+ * it as a KMS framebuffer via &DRM_IOCTL_MODE_ADDFB and map it via
+ * &DRM_IOCTL_MODE_MAP_DUMB.
+ *
+ * &DRM_CAP_DUMB_BUFFER indicates whether this IOCTL is supported.
+ * &DRM_CAP_DUMB_PREFERRED_DEPTH and &DRM_CAP_DUMB_PREFER_SHADOW indicate
+ * driver preferences for dumb buffers.
+ */
 #define DRM_IOCTL_MODE_CREATE_DUMB DRM_IOWR(0xB2, struct drm_mode_create_dumb)
 #define DRM_IOCTL_MODE_MAP_DUMB    DRM_IOWR(0xB3, struct drm_mode_map_dumb)
 #define DRM_IOCTL_MODE_DESTROY_DUMB    DRM_IOWR(0xB4, struct drm_mode_destroy_dumb)

include/uapi/drm/drm_mode.h

@@ -1032,13 +1032,25 @@ struct drm_mode_crtc_page_flip_target {
 	__u64 user_data;
 };
 
-/* create a dumb scanout buffer */
+/**
+ * struct drm_mode_create_dumb - Create a KMS dumb buffer for scanout.
+ * @height: buffer height in pixels
+ * @width: buffer width in pixels
+ * @bpp: bits per pixel
+ * @flags: must be zero
+ * @handle: buffer object handle
+ * @pitch: number of bytes between two consecutive lines
+ * @size: size of the whole buffer in bytes
+ *
+ * User-space fills @height, @width, @bpp and @flags. If the IOCTL succeeds,
+ * the kernel fills @handle, @pitch and @size.
+ */
 struct drm_mode_create_dumb {
 	__u32 height;
 	__u32 width;
 	__u32 bpp;
 	__u32 flags;
-	/* handle, pitch, size will be returned */
+
 	__u32 handle;
 	__u32 pitch;
 	__u64 size;