Sharing Framebuffers between Client / Server

[Thomas Hellstrom]

On 01/20/2014 02:10 PM, Rob Clark wrote:
> On Fri, Jan 17, 2014 at 6:43 AM, Rian Quinn <rianquinn at gmail.com> wrote:
>> I am working on a client/server program, where the server creates (and has
>> access to a framebuffer), and then needs to share this framebuffer with a
>> client program so that this client program can draw into the framebuffer
>> directly (i.e. no memcpy).
>>
>> I am trying to figureout what the “cleanest” way to do this is, such that I
>> can support Intel’s proprietary driver, the open source AMD and NVidia
>> drivers, and the VMWare driver (I have no need for the proprietary
>> ADM/NVidia drivers right now). From what I can tell, GEM is one way to do
>> this. The problem is VMWare doesn’t support GEM.
>>
>> I tried (knowing it would not work), using KMS to create the framebuffer,
>> and then sending the information needed to mmap to the client. This of
>> course failed because the framebuffer is marked non-sharable in the kernel.
> Dmabuf (or just plain old egl/glx which uses dri2 under the hood)
> would probably be what I suggest *except* you mention mmap.  If you
> are doing software rendering, I guess you probably just want to suck
> it up and do XShmPutImage.
>
> >From what I understand, any sort of mmap access to vmwgfx buffers is
> tricky, because they end up being backed by normal GL textures on the
> host OS side (IIUC).  So the single copy upload path in XShmPutImage
> might be close to the ideal path for sw rendered content.
>
>> To be clear, I am fine having to manually write ioctls for each driver, if
>> thats what it takes. But at this point, I am at a loss on the best method to
>> share scannot buffers (or at least in a way that doesn’t make someone cringe
>> when they see my code).
> Some sort of prepare/finish access ioctls for dmabuf to bracket mmap
> access are, I think, what vmwgfx is missing in order to implement
> dmabuf mmap.  But no one so far has needed them badly enough to come
> up with something and send patches.
>
> IIUC, for vmwgfx there would still be a copy back to original texture
> on the host on finish-access, so it might not amount to anything much
> different from XShmPutImage.  Probably better to ask some vmwgfx folks
> to clarify, since the virtual driver has some unique constraints which
> I may not be adequately representing.
>
> BR,
> -R

Rian, for sharing accelerated buffers, They'd best be created with
Mesa's GBM, and shared using DRM prime. Those interfaces are generic and
AFAICT, Ubuntu's Mir works exactly this way. The problem is that the
client would need to link to mesa, and could use GL / GLES to transfer
software contents to the buffer.

For pure software contents, the server would share a generic shared
memory buffer, together with a damage protocol, and composite / copy
onto the framebuffer in the server.

In principle, as Rob says, the dma-buf shared using DRM prime has an
mmap() method, but none of the "big" drivers Intel, Nouveau and Radeon
implements it, and for vmwgfx an implementation would be extremely
inefficient. Also other drivers may have issues with write-combining and
tiling of the mmap'ed framebuffer.

If both the server and client would be 100% software one could create
and share "dumb" kms buffers using DRM prime. If there's something in
the vmwgfx driver that blocks sharing in this way, we could ease that
restriction. But these buffers could not be rendered into.

GEM is, BTW, purely driver-private.

As you might see, the big restriction here is that there is no simple
generic way to mmap() accelerated shared buffers from a lean client.
This is intentional. For vmwgfx it's because of coherency issues that
would make such an implementation inefficient. For other drivers I can
imagine there are tiling- and caching issues.

/Thomas

>
>> Thanks,
>> - Rian
>>
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