DRM DMA Engine

[Jose Abreu]

++ 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?
>
>
>>> Best regards,
>>> Jose Miguel Abreu
>>> _______________________________________________
>>> dri-devel mailing list
>>> dri-devel at lists.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> Best regards,
> Jose Miguel Abreu