Re: Proposal for a low-level Linux display framework

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Alan and Rob,

On Monday 19 September 2011 02:09:36 Rob Clark wrote:
> On Sun, Sep 18, 2011 at 5:23 PM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> >> This would leave us with the issue of controlling formats and other
> >> parameters on the pipelines. We could keep separate DRM, KMS, FB and
> >> V4L APIs for that,
> > 
> > There are some other differences that matter. The exact state and
> > behaviour of memory, sequencing of accesses, cache control and management
> > are a critical part of DRM for most GPUs, as is the ability to have them
> > in swap backed objects and to do memory management of them. Fences and
> > the like are a big part of the logic of many renderers and the same
> > fencing has to be applied between capture and GPU, and also in some cases
> > between playback accelerators (eg MP4 playback) and GPU.

That's why I believe the DRM API is our best solution to address all those 
issues.

I'm not advocating merging the DRM, FB and V4L APIs for memory management. 
What I would like to investigate is whether we can use a common API for the 
common needs, which are (in my opinion):

- reporting the entities that make up the graphics pipeline (such as planes, 
overlays, compositors, transmitters,  connectors, ...), especially when 
pipelines get more complex than the plane->crtc->encoder->connector DRM model

- configuring data routing in those complex pipelines

- and possibly configuring formats (pixel format, frame size, crop rectangle, 
composition rectangle, ...) on those entities

> > To glue them together I think you'd need to support the use of GEM
> > objects (maybe extended) in V4L. That may actually make life cleaner and
> > simpler in some respects because GEM objects are refcounted nicely and
> > have handles.
> 
> fwiw, I think the dmabuf proposal that linaro GWG is working on should
> be sufficient for V4L to capture directly into a GEM buffer that can
> be scanned out (overlay) or composited by GPU, etc, in cases where the
> different dma initiators can all access some common memory:
> 
> http://lists.linaro.org/pipermail/linaro-mm-sig/2011-September/000616.html
> 
> The idea is that you could allocate a GEM buffer, export a dmabuf
> handle for that buffer that could be passed to v4l2 camera device (ie.
> V4L2_MEMORY_DMABUF), video encoder, etc..  the importing device should
> bracket DMA to/from the buffer w/ get/put_scatterlist() so an unused
> buffer could be unpinned if needed.

I second Rob here, I think that API should be enough to solve our memory 
sharing problems between different devices. This is a bit out of scope though, 
as neither the low-level Linux display framework proposal nor my comments 
target that, but it's an important topic worth mentioning.

> > DRM and KMS abstract out stuff into what is akin to V4L subdevices for
> > the various objects the video card has that matter for display - from
> > scanout buffers to the various video outputs, timings and the like.
> > 
> > I don't know what it's like with OMAP but for some of the x86 stuff
> > particularly low speed/power stuff the capture devices, GPU and overlays
> > tend to be fairly incestuous in order to do things like 1080i/p preview
> > while recording from the camera.
> 
> We don't like extra memcpy's, but something like dmabuf fits us
> nicely.. and I expect it would work well in any sort of UMA system
> where camera, encoder, GPU, overlay, etc all can share the same memory
> and formats.  I suspect the situation is similar in the x86 SoC
> world.. but would be good to get some feedback on the proposal.  (I
> guess next version of the RFC would go out to more mailing lists for
> broader review.)
> 
> > GPU is also a bit weird in some ways because while its normally
> > nonsensical to do things like use the capture facility one card to drive
> > part of another, it's actually rather useful (although not supported
> > really by DRM) to do exactly that with GPUs. A simple example is a dual
> > headed box with a dumb frame buffer and an accelerated output both of
> > which are using memory that can be hit by the accelerated card. Classic
> > example being a USB plug in monitor.

-- 
Regards,

Laurent Pinchart