DRM DMA Engine

[Daniel Vetter]

On Thu, Jun 16, 2016 at 01:09:34PM +0100, Jose Abreu wrote:
> Hi Daniel,
> 
> Sorry to bother you again. I promise this is the last time :)
> 
> On 15-06-2016 11:15, Daniel Vetter wrote:
> > On Wed, Jun 15, 2016 at 11:48 AM, Jose Abreu <Jose.Abreu at synopsys.com> wrote:
> >> On 15-06-2016 09:52, Daniel Vetter wrote:
> >>> On Tue, Jun 14, 2016 at 1:19 PM, Jose Abreu <Jose.Abreu at synopsys.com> wrote:
> >>>>> 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.
> >>> DRM has a continuous scanout model, i.e. when userspace doesn't give
> >>> you a new frame you're supposed to keep scanning out the current one.
> >>> So you need to rearm your upload code with the same drm_framebuffer if
> >>> userspace hasn't supplied a new one since the last time before the
> >>> vblank period starts.
> >>>
> >>> This is different to v4l, where userspace has to supply each frame
> >>> (and the kernel gets angry when there's not enough frames and signals
> >>> an underrun of the queue). This is because drm is geared at desktops,
> >>> and there it's perfectly normal to show the exact same frame for a
> >>> long time.
> >>> -Daniel
> >> Thanks, I was thinking this was similar to v4l. I am now able to
> >> send multiple frames so it is finally working! I have one little
> >> implementation detail: The controller that I am using supports
> >> deep color mode but I am using FB CMA helpers to create the
> >> framebuffer and I've seen that the supported bpp in these helpers
> >> only goes up to 32, right? Does this means that with these
> >> helpers I can't use deep color? Can I implement this deep color
> >> mode (48bpp) using a custom fb or do I also need custom gem
> >> allocation functions (Right now I am using GEM CMA helpers)?
> > Suprising the cma doesn't take pixel_format into account. If this
> > really doesn't work, pls fix up the cma helpers, not roll your own
> > copypasta ;-)
> >
> > Note that the fbdev emulation itself (maybe that's what threw you off)
> > only supports legacy rgb formats up to 32bits. But native kms can
> > support anything, we just might need to add the DRM_FOURCC codes for
> > that.
> > -Daniel
> 
> So, I ended up using 32bits and everything is working fine! I
> tested using [1] and [2] but now I have kind of a dumb question:
> I want to use the new driver that I created as a secondary output
> of my desktop so that I can play videos using mplayer but I am
> not being able to do this. If I check in my linux settings only
> one display is being detected, although in /dev/dri the two video
> cards are present (the native one and the one I added). Does the
> driver needs something additional to do this or is it only in my
> X configuration? I tried editing this configuration but still
> doesn't work. I believe that because my driver is not being
> probed at runtime the display is not being created by X. Is this
> correct?

X with multiple drivers is kinda a bit much. I think it should work
somewhat if you treat the 2nd driver as an offload engine. Afaik you can
change that through xrandr, but not sure. I didn't implement this.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch