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

1 files changed, 14 insertions, 2 deletions

diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index ea1b639bcb28..128d09138ceb 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/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;