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drm: document DRM_IOCTL_MODE_GETFB2

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There are a few details specific to the GETFB2 IOCTL.

It's not immediately clear how user-space should check for the
number of planes. Suggest using the handles field or the pitches
field.

The modifier array is filled with zeroes, ie. DRM_FORMAT_MOD_LINEAR.
So explicitly tell user-space to not look at it unless the flag is
set.

Changes in v2 (Daniel):
- Mention that handles should be used to compute the number of planes,
  and only refer to pitches as a fallback.
- Reword bit about undefined modifier.

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211123112400.22245-1-contact@emersion.fr
This commit is contained in:
Simon Ser 2021-11-23 11:24:04 +00:00
parent 1c66496b13
commit 43d5ac7d07
1 changed files with 18 additions and 0 deletions
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18
include/uapi/drm/drm.h
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@ -1096,6 +1096,24 @@ extern "C" {
#define DRM_IOCTL_SYNCOBJ_TRANSFER DRM_IOWR(0xCC, struct drm_syncobj_transfer)
#define DRM_IOCTL_SYNCOBJ_TIMELINE_SIGNAL DRM_IOWR(0xCD, struct drm_syncobj_timeline_array)
/**
* DRM_IOCTL_MODE_GETFB2 - Get framebuffer metadata.
*
* This queries metadata about a framebuffer. User-space fills
* &drm_mode_fb_cmd2.fb_id as the input, and the kernels fills the rest of the
* struct as the output.
*
* If the client is DRM master or has &CAP_SYS_ADMIN, &drm_mode_fb_cmd2.handles
* will be filled with GEM buffer handles. Planes are valid until one has a
* zero handle -- this can be used to compute the number of planes.
*
* Otherwise, &drm_mode_fb_cmd2.handles will be zeroed and planes are valid
* until one has a zero &drm_mode_fb_cmd2.pitches.
*
* If the framebuffer has a format modifier, &DRM_MODE_FB_MODIFIERS will be set
* in &drm_mode_fb_cmd2.flags and &drm_mode_fb_cmd2.modifier will contain the
* modifier. Otherwise, user-space must ignore &drm_mode_fb_cmd2.modifier.
*/
#define DRM_IOCTL_MODE_GETFB2 DRM_IOWR(0xCE, struct drm_mode_fb_cmd2)
/*