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doc: add warning about mapping DMA-BUFs

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A list of reasons why DMA-BUFs have to be treated specially. This
should make it clearer why mmaping DMA-BUFs should be avoided and hint
clients on how treat them correctly

Co-authored-by: columbarius <co1umbarius@protonmail.com>
This commit is contained in:
Simon Ser 2021-08-17 15:02:56 +02:00 • committed by Wim Taymans
parent 112d50c8b9
commit 7a618390ec
1 changed files with 33 additions and 16 deletions
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49
doc/dma-buf.dox
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@ -35,12 +35,12 @@ 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
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
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
@ -48,7 +48,7 @@ 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
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
@ -72,23 +72,23 @@ modifier aware one, or supporting both.
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
(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', followed by the previous announced
'EnumFormat's. This will retrigger (**TBD**) 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
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`, followed by the previous announced
`EnumFormat`s. This will retrigger (**TBD**) 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.
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
`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.
@ -113,4 +113,21 @@ 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.
# 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.
*/