VideoDecoder and DMA-BUF video frames

[Carlos Rafael Giani]

I've been looking at the new VideoDecoder base class which has been
recommended to me as an alternative to the VideoDecodeAccelerator.

So far, this seems very interesting, since the hardware decoder I am
writing this code for does not actually need access to any GPU
resources. Instead, it decodes packetized elementary streams (that is,
it is _not_ slice-based) and writes the decoded frames into DMA-BUF buffers.

Now, I see the OutputCB callback in VideoDecoder. And, VideoFrame has
the WrapExternalDmabufs() function. My question now is: How do I know
when I can reuse a DMA-BUF buffer? In the VDA, there's
ReusePictureBuffer(). The PictureBufferManager in the VDAVideoDecoder
uses sync tokens that tie into the GPU. Is it possible for me to know
when I can reuse a DMA-BUF buffer without these GPU based mechanisms? I
realize that I can dup() a DMA-BUF FD, but with this decoder, I still
have to let it know when it can use a buffer again. That's because the
hardware decoder's driver does not directly use dma-buf. Instead, it
uses physical addresses, and thus does not know anything about buffer
sharing. (There are ways on that platform to get the physical address
from a DMA-BUF buffer, and I know having such addresses is very
insecure, but there's no other way on that platform, unfortunately.) So,
I must let the hw decoder know when Chromium is done with that picture.

On a related note, this also means that the buffer must not be reused by
the hw decoder until the GPU itself is fully done with that picture.
When I integrated that hw decoder using a VDA, I added code to manually
use an EGL fence to not return the picture until the EGL sync object was
signaled. With DMA-BUF buffers, do I still have to do something like that?

cheers

[Dale Curtis]

+Dan Sanders +Andres Calderon Jaramillo who may have some insight. You may also try looking through the VaapiVideoDecoder and V4L2VideoDecoder classes which utilize (or at least used to) DMA-BUF.

- dale

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[Dan Sanders]

How do I know
when I can reuse a DMA-BUF buffer?

If you set read_lock_fences_enabled, VideoFrame consumers are required to synchronize reads before dropping their VideoFrame reference. You'll see this approach used wherever SyncTokens are insufficient.

[Carlos Rafael Giani]

And can I get a notification for when all consumers have dropped their VideoFrame references?

[Dan Sanders]

If you have mailboxes then you should use the mailbox_holders_release_cb, otherwise use AddDestructionObserver().