A few questions about the best way to implement RandR 1.4 / PRIME buffer sharing

[Dave Airlie]

> object interface, so that Optimus-based laptops can use our driver to drive
> the
> discrete GPU and display on the integrated GPU.  The good news is that I've
> got
> a proof of concept working.

Don't suppose you'll be interested in adding the other method at some
point as well? since saving power is probably important to a lot of
people :-)

>
> During a review of the current code, we came up with a few concerns:
>
> 1. The output source is responsible for allocating the shared memory
>
> Right now, the X server calls CreatePixmap on the output source screen and
> then
> expects the output sink screen to be able to display from whatever memory
> the
> source allocates.  Right now, the source has no mechanism for asking the
> sink
> what its requirements are for the surface.  I'm using our own internal pitch
> alignment requirements and that seems to be good enough for the Intel device
> to
> scan out, but that could be pure luck.

Well in theory it might be nice but it would have been premature since
so far the only interactions for prime are combination of intel,
nvidia and AMD, and I think everyone has fairly similar pitch
alignment requirements, I'd be interested in adding such an interface
but I don't think its some I personally would be working on.

> other, or is it sufficient to just define a lowest common denominator format
> and
> if your hardware can't deal with that format, you just don't get to share
> buffers?

At the moment I'm happy to just go with linear, minimum pitch
alignment 64 or something as a base standard, but yeah I'm happy for
it to work either way, just don't have enough evidence it's worth it
yet. I've not looked at ARM stuff, so patches welcome if people
consider they need to use this stuff for SoC devices.

> 2. There's no fallback mechanism if sharing can't be negotiated
>
> If RandR fails to share a pixmap with the output sink screen, the whole
> modeset
> fails.  This means you'll end up not seeing anything on the screen and
> you'll
> probably think your computer locked up.  Should there be some sort of
> software
> copy fallback to ensure that something at least shows up on the display?

Uggh, it would be fairly slow and unuseable, I'd rather they saw
nothing, but again open to suggestions on how to make this work, since
it might fail for other reasons and in that case there is still
nothing a sw copy can do. What happens if the slave intel device just
fails to allocate a pixmap, but yeah I'm willing to think about it a
bit more when we have some reference implementations.
>
> 3. How should the memory be allocated?
>
> In the prototype I threw together, I'm allocating the shared memory using
> shm_open and then exporting that as a dma-buf file descriptor using an ioctl
> I
> added to the kernel, and then importing that memory back into our driver
> through
> dma_buf_attach & dma_buf_map_attachment.  Does it make sense for user-space
> programs to be able to export shmfs files like that?  Should that interface
> go
> in DRM / GEM / PRIME instead?  Something else?  I'm pretty unfamiliar with
> this
> kernel code so any suggestions would be appreciated.

Your kernel driver should in theory be doing it all, if you allocate
shared pixmaps in GTT accessible memory, then you need an ioctl to
tell your kernel driver to export the dma buf to the fd handle.
(assuming we get rid of the _GPL, which people have mentioned they are
open to doing). We have handle->fd and fd->handle interfaces on DRM,
you'd need something similiar on the nvidia kernel driver interface.

Yes for 4 some sort of fencing is being worked on by Maarten for other
stuff but would be a pre-req for doing this, and also some devices
don't want fullscreen updates, like USB, so doing flipped updates
would have to be optional or negoitated. It makes sense for us as well
since things like gnome-shell can do full screen pageflips and we have
to do full screen dirty updates.

Dave.