[PATCH] drm/doc: document DRM_IOCTL_MODE_CREATE_DUMB

* [PATCH] drm/doc: document DRM_IOCTL_MODE_CREATE_DUMB
@ 2023-08-03 10:00 Simon Ser
  2023-08-21  8:49 ` Pekka Paalanen
  0 siblings, 1 reply; 2+ messages in thread
From: Simon Ser @ 2023-08-03 10:00 UTC (permalink / raw)
  To: dri-devel; +Cc: Daniel Vetter, Pekka Paalanen

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.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
---
 Documentation/gpu/drm-kms.rst |  2 ++
 include/uapi/drm/drm.h        | 15 +++++++++++++++
 include/uapi/drm/drm_mode.h   | 16 ++++++++++++++--
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
index c92d425cb2dd..ca9210e47266 100644
--- a/Documentation/gpu/drm-kms.rst
+++ b/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
 ===================
 
diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
index 863e47200911..625dba7ddbfe 100644
--- a/include/uapi/drm/drm.h
+++ b/include/uapi/drm/drm.h
@@ -1128,6 +1128,21 @@ 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 without KMS. 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.
+ */
 #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)
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 92d96a2b6763..8a7e747f0564 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -1029,13 +1029,25 @@ 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;
-- 
2.41.0



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