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doc: add section about v4l2 dmabuf sharing

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columbarius 2021-08-17 15:57:25 +02:00 committed by Wim Taymans
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doc/dma-buf.dox
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@ -5,6 +5,9 @@ clients via the `SPA_DATA_DmaBuf` data type. However properly negotiating
DMA-BUF support on both the producer and the consumer side require following
a specific procedure. This page describes said procedure by using events and
methods from the filter or stream API.
Note: This article focuses mostly on DMA-BUF sharing from arbitrary devices,
like discrete GPUs. For using DMA-BUFs created by v4l2 please refer to the
corresponding paragraph.
# Capability negotiations
@ -130,4 +133,17 @@ because of the following issues:
using a proper graphics API (such as EGL, Vulkan or VA-API) to process the
DMA-BUFs.
# v4l2
Another use case for streaming via DMA-BUFs are exporting a camera feed from v4l2
as DMA-BUFs. Those are located in the main memory where it is possible to mmap them.
This should be done as follows: Neither producer nor consumer should announce a
modifier, but both should include `1 << SPA_DATA_DmaBuf` in the
`SPA_PARAM_BUFFERS_dataType` property. It's the the responsibility of the producer
while the `add_buffer` event to choose DMA-BUF as the used buffer type even though
no modifier is present, if it can guarantee, that the used buffer is mmapable.
Note: For now v4l2 uses planar buffers without modifiers. This is the reason for
this special case.
*/