linux-kernel - Re: DRM DMA Engine

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Message-ID: <576296FE.4030504@synopsys.com>
Date:	Thu, 16 Jun 2016 13:09:34 +0100
From:	Jose Abreu <Jose.Abreu@...opsys.com>
To:	Daniel Vetter <daniel@...ll.ch>,
	Jose Abreu <Jose.Abreu@...opsys.com>
CC:	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>
Subject: Re: DRM DMA Engine

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@...opsys.com> wrote:
>> On 15-06-2016 09:52, Daniel Vetter wrote:
>>> On Tue, Jun 14, 2016 at 1:19 PM, Jose Abreu <Jose.Abreu@...opsys.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?

[1] https://dri.freedesktop.org/libdrm/
[2] https://github.com/dvdhrm/docs/blob/master/drm-howto/modeset.c

Thanks!

Best regards,
Jose Miguel Abreu