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dmabuf: Make it clear that the FORMAT is a DRM fourcc

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Some of the formats in GstVideoFormat are a combination
of a DRM format and modifier, making them unclear.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2616>
This commit is contained in:
Olivier Crête 2022-06-15 17:32:06 -04:00 committed by GStreamer Marge Bot
parent 6c4c576699
commit 96422cc9cf
1 changed files with 22 additions and 13 deletions
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35
subprojects/gst-docs/markdown/additional/design/dmabuf.md
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@ -49,7 +49,8 @@ Just like fourcc common usage, DRM-fourcc describes the underlying format
of the video frame, such as `DRM_FORMAT_YVU420` or `DRM_FORMAT_NV12`. All
of them with the prefix `DRM_FORMAT_`. Please refer to `drm_fourcc.h` in
the kernel for a full list. This list of fourcc formats maps to GStreamer
video formats.
video formats, although the GStreamer formats may have a slighly different.
For example, DRM_FORMAT_ARGB8888 corresponds to GST_VIDEO_FORMAT_BGRA.
## DRM modifier
@ -115,12 +116,20 @@ sinks might end rendering wrong frames assuming linear access.
Because DRM-fourcc and DRM-modifier are both necessary to render frames
DMABuf-backed, we now consider both as a pair and combine them together to
assure uniqueness. In caps, we use a *:* to link them together and write in
the mode of *FORMAT:MODIFIER*, which represents a totally new single video
the mode of *DRM_FORMAT:DRM_MODIFIER*, which represents a totally new single video
format. For example, `NV12:0x0100000000000002` is a new video format
combined by video format NV12 and the modifier `0x0100000000000002`. It's
not NV12 and it's not its subset either. A modifier must always be present,
except if the modifier is linear, then it should not be included,
so `NV12:0x0000000000000000` is invalid, it must be `drm-format=NV12`.
not NV12 and it's not its subset either.
*DRM_FORMAT* can be printed by using
`GST_FOURCC_FORMAT` and `GST_FOURCC_ARGS` macros from the
`DRM_FORMAT_*` constants, it is NOT a `GstVideoFormat`, so it would be
different from the content of the `format` field in a non-dmabuf caps.
A modifier must always be present, except if the modifier is linear,
then it should not be included, so `NV12:0x0000000000000000` is
invalid, it must be `drm-format=NV12`. DRM fourcc are used
instead of a `GstVideoFormat` to make it easier for non-GStreamer
developers to understand what the system is trying to achieve.
Please note that this form of video format only appears within
*memory:DMABuf* feature. It must not appear in any other video caps
@ -209,7 +218,7 @@ DMA buffer, **unless the modifier is linear**.
If two elements of different modules (for example, VA-API decoder to
Wayland sink) want to transfer dmabufs, the negotiation should ensure a
common *drm-format* (FORMAT:MODIFIER). As we already illustrate how to
common *drm-format* (*DRM_FORMAT:DRM_MODIFIER*). As we already illustrate how to
represent both of them in caps before, so the negotiation here in fact has
no special operation except finding the intersection.
@ -227,13 +236,13 @@ SRC template: 'src'
width: [ 16, 16384 ]
height: [ 16, 16384 ]
drm-format: { (string)NV12:0x0100000000000001, \
(string)I420, (string)YV12, \
(string)YUY2:0x0100000000000002, \
(string)P010_10LE:0x0100000000000002, \
(string)BGRA:0x0100000000000002, \
(string)RGBA:0x0100000000000002, \
(string)BGR10A2_LE:0x0100000000000002, \
(string)VUYA:0x0100000000000002 }
(string)YU12, (string)YV12, \
(string)YUYV:0x0100000000000002, \
(string)P010:0x0100000000000002, \
(string)AR24:0x0100000000000002, \
(string)AB24:0x0100000000000002, \
(string)AR39:0x0100000000000002, \
(string)AYUV:0x0100000000000002 }
```
But because sometimes it is impossible to enumerate and list all