Update front buffer without CPU interaction?

[Volker Vogelhuber <v.vogelhuber@digitalendoscopy.de>]

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.

Kind regards,
     Volker


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