DRM DMA Engine

[Jose Abreu]

Hi Ilia,

Thanks for your answer.

On 16-06-2016 13:39, Ilia Mirkin wrote:
> On Thu, Jun 16, 2016 at 8:09 AM, Jose Abreu <Jose.Abreu at synopsys.com> 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?
> Have a look at
>
> https://nouveau.freedesktop.org/wiki/Optimus/
>
> specifically the section titled "Using outputs on discrete GPU". If
> you're still having trouble, please provide an Xorg log.

I found another solution: using mpv I am able to specify which
output card I can use (in my case its /dev/dri/card1). Although,
I only can do this from command line. If I start the driver from
graphical mode the link between crtc and encoder disappears and
if I try to run mpv it fails to find the crtc for the given
encoder, but if I start and run mpv from command line everything
works fine. Also, when I do 'startx' from command line the X
server fails, giving this log:

[snip]
[  4815.102] (II) xfree86: Adding drm device (/dev/dri/card1)
[  4815.127] (EE)
[  4815.132] (EE) Backtrace:
[  4815.157] (EE) 0: /usr/bin/X (xorg_backtrace+0x56)
[0x7f9de4f255a6]
[  4815.162] (EE) 1: /usr/bin/X (0x7f9de4d72000+0x1b7709)
[0x7f9de4f29709]
[  4815.167] (EE) 2: /lib/x86_64-linux-gnu/libc.so.6
(0x7f9de2a35000+0x352f0) [0x7f9de2a6a2f0]
[  4815.172] (EE) 3: /usr/bin/X (0x7f9de4d72000+0xb9899)
[0x7f9de4e2b899]
[  4815.177] (EE) 4: /usr/bin/X (xf86BusProbe+0x9) [0x7f9de4dfe249]
[  4815.181] (EE) 5: /usr/bin/X (InitOutput+0x734) [0x7f9de4e0cd54]
[  4815.183] (EE) 6: /usr/bin/X (0x7f9de4d72000+0x5c0ba)
[0x7f9de4dce0ba]
[  4815.184] (EE) 7: /lib/x86_64-linux-gnu/libc.so.6
(__libc_start_main+0xf0) [0x7f9de2a55a40]
[  4815.185] (EE) 8: /usr/bin/X (_start+0x29) [0x7f9de4db8639]
[  4815.186] (EE)
[  4815.187] (EE) Segmentation fault at address 0x0
[  4815.188] (EE)
Fatal server error:
[  4815.191] (EE) Caught signal 11 (Segmentation fault). Server
aborting
[  4815.192] (EE)
[  4815.193] (EE)
Please consult the The X.Org Foundation support
     at http://wiki.x.org
 for help.
[  4815.197] (EE) Please also check the log file at
"/var/log/Xorg.1.log" for additional information.
[  4815.198] (EE)

Do you know of any reason why this might be happening? Notice
that from command line everything works fine.

Best regards,
Jose Miguel Abreu