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?
Regards, Volker
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