Update front buffer without CPU interaction?

[Daniel Vetter]

On Mon, Feb 23, 2015 at 09:22:56AM +0100, Volker Vogelhuber wrote:
> 
> On 22.02.2015 12:52, Daniel Vetter wrote:
> >On Mon, Feb 16, 2015 at 01:43:07PM +0100, Volker Vogelhuber wrote:
> >>I'm currently trying to setup a rendering pipe on an Intel Baytrail E3845
> >>cpu.
> >>In our product we want to have an FPGA streaming video images to a
> >>predefined memory area using bus master dma and render those images using
> >>OpenGL. So far this works in a preliminary state.
> >>We now have the security requirement that in case the CPU (software/kernel
> >>driver) crashes for what ever reason, the GPU display signal should still
> >>output at least the video images (obviously any additional render stuff will
> >>not be available anymore). My question is now, would it be possible to get
> >>the physical address of the DRM front buffer, so that I can provide this
> >>address to the FPGA (connected via PCIe) and is it possible to have the GPU
> >>still reading the last front buffer for the display output while the FPGA
> >>writes to that area. So I would think that the GPU has some kind of DMA
> >>engine running, that continuously reading the last front buffer until
> >>switched to another buffer by the CPU. So even if the CPU does not control
> >>the GPU anymore, it might be possible to have the front buffer updated by
> >>the FPGA directly. Of course there will be tearing artefacts as no VSYNC
> >>will be available but that wouldn't be an issue so far.
> >Just share buffers between your fpga driver and the i915 driver using
> >prime/dma-buf and before each pageflip tell your fpga driver to render
> >into the new buffer. We don't have any interfaces to tell userspace
> >physical addresses of anything, and that's fairly intentional - the kernel
> >is and must be in control of memory management.
> >-Daniel
> Thanks for the reply. I already stumbled across the PRIME way to get access
> to the buffer within the source code but it wasn't totally clear to me, how
> one would access this API.
> I guess one have to call drmPrimeHandleToFD with the /dev/dri/card0 file
> descriptor, a gem handle (probably retrieved using gbm_bo_get_handle),
> DRM_CLOEXEC and some output variable.
> The Prime file descriptor returned, should be usable with the dma-buf API,
> right? So I can call dma_buf_get(prime_fd) within the FPGA driver and pass
> the sg_table I get from dma_buf_map_attachment to the DMA engine within the
> FPGA.
> Does this concept work in theory? Or did I miss something?

It should work even in practice ;-) And yeah description of the dance
looks complete.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch