DRM DMA Engine

[Jose Abreu]

Hi Daniel,


On 30-05-2016 10:36, Daniel Vetter wrote:
> On Mon, May 30, 2016 at 10:00:56AM +0100, Jose Abreu wrote:
>> ++ Daniel
>>
>>
>> On 30-05-2016 09:44, Jose Abreu wrote:
>>> Hi Daniel,
>>>
>>> Thanks for your answer.
>>>
>>> On 26-05-2016 09:06, Daniel Vetter wrote:
>>>> On Wed, May 25, 2016 at 04:46:15PM +0100, Jose Abreu wrote:
>>>>> Hi all,
>>>>>
>>>>> Currently I am trying to develop a DRM driver that will use
>>>>> Xilinx VDMA to transfer video data to a HDMI TX Phy and I am
>>>>> facing a difficulty regarding the understanding of the DRM DMA
>>>>> Engine. I looked at several sources and at the DRM core source
>>>>> but the flow of creating and interfacing with the DMA controller
>>>>> is still not clear to me.
>>>>>
>>>>> At DRI web page the X server is mentioned. Does it mean that the
>>>>> channel creation and handling is done by the X server? If so,
>>>>> what is the DRM driver responsible to do then and what exactly
>>>>> does the DRM core do? As I am using Xilinx VDMA do you foresee
>>>>> any special implementation details?
>>>>>
>>>>> Just for reference here is the description of the Xilinx VDMA:
>>>>> "The Advanced eXtensible Interface Video Direct Memory Access
>>>>> (AXI VDMA) core is a soft Xilinx Intellectual Property (IP) core
>>>>> providing high-bandwidth direct memory access between memory and
>>>>> AXI4-Stream video type target peripherals including  peripherals
>>>>> which support AXI4-Stream Video Protocol." The driver is
>>>>> available at "drivers/dma/xilinx/xilinx_vdma.c".
>>>>>
>>>>> Another important point: I am using PCI Express connected to a
>>>>> FPGA which has all the necessary components (Xilinx VDMA, I2S,
>>>>> ...) and the HDMI TX Phy.
>>>>>
>>>>> Looking forward to you help.
>>>> If your dma engine is just for HDMI display, forget all the stuff you find
>>>> about DRI and X server on the various wikis. That's for opengl rendering.
>>>>
>>>> The only thing you need is a kernel-modesetting driver, and nowadays those
>>>> are written using the atomic modeset framework. There's plenty of
>>>> introductory talks and stuff all over the web (I suggest the latest
>>>> version of Laurent Pinchart's talk as a good starting point).
>>>> -Daniel
>>> I watched the talk of Laurent and I already have a simple KMS
>>> driver with an encoder (which is bridge dw-hdmi), a connector and
>>> a crtc. My doubt now is how do I setup the video path so that
>>> video samples are sent using the Xilinx VDMA to our hdmi phy.
>>>
>>> Sorry if I am making some mistake (I am quite new to DRM and DMA)
>>> but here is my thoughts:
>>>     - A DMA channel or some kind of mapping must be done so that
>>> the DRM driver knows where to send samples;
>>>     - The Xilinx VDMA driver must be instantiated (which I am
>>> already doing);
>>>     - Some kind of association between the DRM DMA engine and
>>> Xilinx VDMA must be done;
>>>     - A callback should exist that is called on each frame and
>>> updates the data that is sent to Xilinx VDMA.
>>>
>>> Does this looks okay to you or am I missing something? I still
>>> haven't figured out how should I associate the VDMA to the DRM
>>> DMA engine and how should I map the DMA to the DRM driver.
>>>
>>> Can you give me some help or refer me to someone who can? Also,
>>> is there a DRM driver that uses a similar architecture?
> I assume that xilinx VDMA is the only way to feed pixel data into your
> display pipeline. Under that assumption:
>
> drm_plane should map to Xilinx VDMA, and the drm_plane->drm_crtc link
> would represent the dma channel. With atomic you can subclass
> drm_plane/crtc_state structures to store all the runtime configuration in
> there.
>
> The actual buffer itsel would be represented by a drm_framebuffer, which
> either wraps a shmem gem or a cma gem object.
>
> If you want to know about the callbacks used by the atomic helpers to push
> out plane updates, look at the hooks drm_atomic_helper_commit_planes()
> (and the related functions, see kerneldoc) calls.
>
> I hope this helps a bit more.
> -Daniel

Thanks a lot! With your help I was able to implement all the
needed logic. Sorry to bother you but I have one more question.
Right now I can initialize and configure the vdma correctly but I
can only send one frame. I guess when the dma completes
transmission I need to ask drm for a new frame, right? Because
the commit function starts the vdma correctly but then the dma
halts waiting for a new descriptor.

Best regards,
Jose Miguel Abreu