PipeWire: DMA-BUF Sharing

PipeWire 1.2.0

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DMA-BUF Sharing

PipeWire supports sharing Direct Memory Access buffers (DMA-BUFs) between 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

The capability negotiation for DMA-BUFs is complicated by the fact that a usable and preferred optimal modifier for a given format can only be determined by the allocator. This allocator has to be invoked with the intersection of all supported modifiers for every client. As a result, the fixation of the modifier is delegated from PipeWire to the node responsible for allocating the buffers.

## pw_stream_connect

The stream parameters should contain two SPA_PARAM_EnumFormat objects for each format: one for DMA-BUFs, one for shared memory buffers as a fallback.

Query the list of all supported modifiers from your graphics API of choice. Add a SPA_FORMAT_VIDEO_modifier property to the first stream parameter with the flags `SPA_POD_PROP_FLAG_MANDATORY | SPA_POD_PROP_FLAG_DONT_FIXATE`. The value of the property should be set to a SPA_CHOICE_Enum containing one `long` choice per supported modifier, plus `DRM_FORMAT_MOD_INVALID` if the graphics API supports modifier-less buffers.

Note: When a producer is only supporting modifier-less buffers it can omit the SPA_POD_PROP_FLAG_DONT_FIXATE (see param_changed hook, For producers).

The second stream parameter should not contain any SPA_FORMAT_VIDEO_modifier property.

To prioritise DMA-BUFs place those SPA_PARAM_EnumFormat containing modifiers first, when emitting them to PipeWire.

## param_changed Hook

When the `param_changed` hook is called for a SPA_PARAM_Format the client has to parse the `spa_pod` directly. Use `spa_pod_find_prop(param, NULL, SPA_FORMAT_VIDEO_modifier)` to check whether modifiers were negotiated. If they were negotiated, set the SPA_PARAM_BUFFERS_dataType property to `1 << SPA_DATA_DmaBuf`. If they were not negotiated, fall back to shared memory by setting the SPA_PARAM_BUFFERS_dataType property to `1 << SPA_DATA_MemFd`, `1 << SPA_DATA_MemPtr`, or both.

While consumers only have to parse the resulting SPA_PARAM_Format for any format related information, it's up to the producer to fixate onto a single format modifier pair. The producer is also responsible to check if all clients announce sufficient capabilities or fallback to shared memory buffers when possible.

### For Consumers

Use spa_format_video_raw_parse to get the format and modifier.

### For Producers

Producers have to handle two cases when it comes to modifiers wrt. to the previous announced capabilities: Using only the modifier-less API, only the modifier-aware one, or supporting both.

- modifier-less: In this case only the modifier `DRM_FORMAT_MOD_INVALID` was announced with the format. It is sufficient to check if the SPA_PARAM_Format contains the modifier property as described above. If that is the case, use DMA-BUFs for screen-sharing, else fall back to SHM, if possible.
- modifier-aware: In this case a list with all supported modifiers will be returned in the format. (using `DRM_FORMAT_MOD_INVALID` as the token for the modifier-less API). On the `param_changed` event check if the modifier key is present and has the flag SPA_POD_PROP_FLAG_DONT_FIXATE attached to it. In this case, extract all modifiers from the list and do a test allocation with your allocator to choose the preferred modifier. Fixate on that EnumFormat by announcing a SPA_PARAM_EnumFormat with only one modifier in the SPA_CHOICE_Enum and without the SPA_POD_PROP_FLAG_DONT_FIXATE flag, followed by the previous announced EnumFormat. This will retrigger the `param_changed` event with an SPA_PARAM_Format as described below. If the SPA_PARAM_Format contains a modifier key, without the flag SPA_POD_PROP_FLAG_DONT_FIXATE, it should only contain one value in the SPA_CHOICE_Enum. In this case announce the SPA_PARAM_Buffers accordingly to the selected format and modifier. It is important to query the plane count of the used format modifier pair and set `SPA_PARAM_BUFFERS_blocks` accordingly.

Note: When test allocating a buffer, collect all possible modifiers, while omitting `DRM_FORMAT_MOD_INVALID` from the SPA_FORMAT_VIDEO_modifier property and pass them all to the graphics API. If the allocation fails and the list of possible modifiers contains `DRM_FORMAT_MOD_INVALID`, fall back to allocating without an explicit modifier if the graphics API allows it.

## add_buffer Hook

This is relevant for producers.

Allocate a DMA-BUF only using the negotiated format and modifier.

## on_event Hook

This is relevant for consumers.

Check the type of the dequeued buffer. If its SPA_DATA_MemFd or SPA_DATA_MemPtr use the fallback SHM import mechanism. If it's SPA_DATA_DmaBuf get the DMA-BUF FDs (the plane count is encoded in the `n_datas` variable of the `spa_buffer` struct) and import them with the graphics API.

Note: Some graphics APIs have separated functions for the modifier-less case (`DRM_FORMAT_MOD_INVALID`) or are omitting the modifier, since it might be used for error handling.

## Example Programs

- video-src-fixate.c:
  Fixating negotiated modifiers.

- video-play-fixate.c:
  Video input stream using pw_stream, with format fixation.

# DMA-BUF Mapping Warning

It's important to make sure all consumers of the PipeWire stream are prepared to deal with DMA-BUFs. Most DMA-BUFs cannot be treated like shared memory in general because of the following issues:

- DMA-BUFs can use hardware-specific tiling and compression as described by modifiers. Thus, a `mmap(3)` on the DMA-BUF FD will not give a linear view of the buffer contents.
- DMA-BUFs need to be properly synchronized with the asynchronous reads and writes of the hardware. A `mmap(3)` call is not enough to guarantee proper synchronization. (Maybe add link to linux syscall doc??)
- Blindly accessing the DMA-BUFs via `mmap(3)` can be extremely slow if the buffer has been allocated on discrete hardware. Consumers are better off using a proper graphics API (such as EGL, Vulkan or VA-API) to process the DMA-BUFs.

# Size of DMA-BUFs

When importing a DMA-BUF with a proper graphics API the size of a single buffer plane is no relevant property since it will be derived by the driver from the other properties. Therefore consumers should ignore the field `maxsize` of a `spa_data` and the field `size` of a `spa_chunk` struct. Producers are allowed to set both to 0. In cases where mapping a single plane is required the size should be obtained locally via the filedescriptor.

# SPA param video format helpers

SPA offers helper functions to parse and build a spa_pod object to/from the spa_video_info_* struct. The flags SPA_VIDEO_FLAG_MODIFIER and SPA_VIDEO_FLAG_MODIFIER_FIXATION_REQUIRED are used to indicate modifier usage with the format. `SPA_VIDEO_FLAG_MODIFIER` declares the parsed/provided spa_video_info_* struct contains valid modifier information. For legacy reasons `spa_format_video_*_build` will announce any modifier != 0 even when this flag is unused. `SPA_VIDEO_FLAG_MODIFIER_FIXATION_REQUIRED` is exclusive to the parse helpers and declares that the parsed spa_pod contains modifier information which needs to be fixated as described above. The list of available modifiers has to be parsed manually from the spa_pod object.

- spa_video_info_raw, spa_format_video_raw_parse, spa_format_video_raw_build
- spa_video_info_dsp, spa_format_video_dsp_parse, spa_format_video_dsp_build

# 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.