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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 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 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 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 ## 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 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 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 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 format. For example, `NV12:0x0100000000000002` is a new video format
combined by video format NV12 and the modifier `0x0100000000000002`. It's 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, not NV12 and it's not its subset either.
except if the modifier is linear, then it should not be included,
so `NV12:0x0000000000000000` is invalid, it must be `drm-format=NV12`. *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 Please note that this form of video format only appears within
*memory:DMABuf* feature. It must not appear in any other video caps *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 If two elements of different modules (for example, VA-API decoder to
Wayland sink) want to transfer dmabufs, the negotiation should ensure a 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 represent both of them in caps before, so the negotiation here in fact has
no special operation except finding the intersection. no special operation except finding the intersection.
@ -227,13 +236,13 @@ SRC template: 'src'
width: [ 16, 16384 ] width: [ 16, 16384 ]
height: [ 16, 16384 ] height: [ 16, 16384 ]
drm-format: { (string)NV12:0x0100000000000001, \ drm-format: { (string)NV12:0x0100000000000001, \
(string)I420, (string)YV12, \ (string)YU12, (string)YV12, \
(string)YUY2:0x0100000000000002, \ (string)YUYV:0x0100000000000002, \
(string)P010_10LE:0x0100000000000002, \ (string)P010:0x0100000000000002, \
(string)BGRA:0x0100000000000002, \ (string)AR24:0x0100000000000002, \
(string)RGBA:0x0100000000000002, \ (string)AB24:0x0100000000000002, \
(string)BGR10A2_LE:0x0100000000000002, \ (string)AR39:0x0100000000000002, \
(string)VUYA:0x0100000000000002 } (string)AYUV:0x0100000000000002 }
``` ```
But because sometimes it is impossible to enumerate and list all But because sometimes it is impossible to enumerate and list all