Update front buffer without CPU interaction?

[Volker Vogelhuber]

On 23.02.2015 10:18, Daniel Vetter wrote:
> 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.
>
>
I haven't implemented all of the proposed flow (only did a quick writing 
test into the framebuffer from within the FPGA driver using the CPU) but 
it worked more or less without any problems when using an intel 
baytrail. On an AMD G-Series GX-412HC the API seems to be implemented as 
well, but during the call of dma_buf_map_attachment there is a NULL 
pointer exception within the call to radeon_gem_prime_get_sg_table. It 
seems like bo->tbo is a null pointer (used kernel 3.19). Do I have to do 
anything differently when using the radeon driver, or isn't it supported 
at all for this card?