Try to address the DMA-buf coherency problem

[Pekka Paalanen]

On Wed, 2 Nov 2022 12:18:01 +0100
Christian König <ckoenig.leichtzumerken at gmail.com> wrote:

> Am 01.11.22 um 22:09 schrieb Nicolas Dufresne:
> > [SNIP]  
> >>> But the client is just a video player. It doesn't understand how to
> >>> allocate BOs for Panfrost or AMD or etnaviv. So without a universal
> >>> allocator (again ...), 'just allocate on the GPU' isn't a useful
> >>> response to the client.  
> >> Well exactly that's the point I'm raising: The client *must* understand
> >> that!
> >>
> >> See we need to be able to handle all restrictions here, coherency of the
> >> data is just one of them.
> >>
> >> For example the much more important question is the location of the data
> >> and for this allocating from the V4L2 device is in most cases just not
> >> going to fly.  
> > It feels like this is a generic statement and there is no reason it could not be
> > the other way around.  
> 
> And exactly that's my point. You always need to look at both ways to 
> share the buffer and can't assume that one will always work.
> 
> As far as I can see it you guys just allocate a buffer from a V4L2 
> device, fill it with data and send it to Wayland for displaying.
> 
> To be honest I'm really surprised that the Wayland guys hasn't pushed 
> back on this practice already.

What should we Wayland people be pushing back on exactly? And where is
our authority and opportunity to do so?

The Wayland protocol dmabuf extension allows a graceful failure if the
Wayland compositor cannot use the given dmabuf at all, giving the
client an opportunity to try something else. The Wayland protocol also
tells clients which DRM rendering device at minimum the dmabuf needs to
be compatible with. It even tells which KMS device the dmabuf could be
put on direct scanout if the dmabuf was suitable for that and direct
scanout is otherwise possible.

What the client (application) does with all that information is up to
the client. That code is not part of Wayland.

I'm sure we would be happy to add protocol for anything that
https://github.com/cubanismo/allocator needs to become the universal
optimal buffer allocator library.


> This only works because the Wayland as well as X display pipeline is 
> smart enough to insert an extra copy when it find that an imported 
> buffer can't be used as a framebuffer directly.

The only fallback Wayland compositors tend to do is use OpenGL/Vulkan
rendering for composition if direct scanout on a KMS plane does not
work. There are many reasons why direct scanout may not be possible in
addition to hardware and drivers not agreeing to do it with the given
set of buffers.

A general purpose (read: desktop) Wayland compositor simply cannot live
without being able to fall back from KMS composition to software/GPU
composition.

But yes, there are use cases where that fallback is as good as failing
completely. Those are not desktops but more like set-top-boxes and TVs.


Thanks,
pq
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