Le jeudi 08 novembre 2018 à 16:45 +0900, Tomasz Figa a écrit :
> > In this patch we should consider a way to tell userspace that this has
> > been opt in, otherwise existing userspace will have to remain using
> > sub-optimal copy based reclaiming in order to ensure that renegotiation
> > can work on older kernel tool. At worst someone could probably do trial
> > and error (reqbufs(1)/mmap/reqbufs(0)) but on CMA with large buffers
> > this introduces extra startup time.
> 
> Would such REQBUFS dance be really needed? Couldn't one simply try
> reqbufs(0) when it's really needed and if it fails then do the copy,
> otherwise just proceed normally?

In simple program, maybe, in modularized code, where the consumer of
these buffer (the one that is forced to make a copy) does not know the
origin of the DMABuf, it's a bit complicated.

In GStreamer as an example, the producer is a plugin called
libgstvideo4linux2.so, while the common consumer would be libgstkms.so.
They don't know each other. The pipeline would be described as:

  v4l2src ! kmssink

GStreamer does not have an explicit reclaiming mechanism. No one knew
about V4L2 restrictions when this was designed, DMABuf didn't exist and
GStreamer didn't have OMX support.

What we ended up crafting, as a plaster, is that when upstream element
(v4l2src) query a new allocation from downstream (kmssink), we always
copy and return any ancient buffers by copying. kmssink holds on a
buffer because we can't remove the scannout buffer on the display. This
is slow and inefficient, and also totally unneeded if the dmabuf
originate from other kernel subsystems (like DRM).

So what I'd like to be able to do, to support this in a more optimal
and generic way, is to mark the buffers that needs reclaiming before
letting them go. But for that, I would need a flag somewhere to tell me
this kernel allow this.

You got the context, maybe the conclusion is that I should simply do
kernel version check, though I'm sure a lot of people will backport
this, which means that check won't work so well.

Let me know, I understand adding more API is not fun, but as nothing is
ever versionned in the linux-media world, it's really hard to detect
and use new behaviour while supporting what everyone currently run on
their systems.

I would probably try and find a way to implement your suggestion, and
then introduce a flag in the query itself, but I would need to think
about it a little more. It's not as simple as it look like
unfortunately.

Nicolas