Re: DRM DMA Engine

From: Daniel Vetter
Date: Mon May 30 2016 - 05:36:52 EST

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

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 Vetter
Software Engineer, Intel Corporation