Implementing Miracast?

[Martin Peres]

On 03/12/15 18:38, Ilia Mirkin wrote:
> On Thu, Dec 3, 2015 at 11:10 AM, Laurent Pinchart
> <laurent.pinchart at ideasonboard.com> wrote:
>> Hi Ilia,
>>
>> On Thursday 03 December 2015 11:03:28 Ilia Mirkin wrote:
>>> On Thu, Dec 3, 2015 at 10:53 AM, Laurent Pinchart wrote:
>>>> On Thursday 03 December 2015 10:42:50 Ilia Mirkin wrote:
>>>>> On Thu, Dec 3, 2015 at 10:34 AM, Laurent Pinchart wrote:
>>>>>> On Thursday 03 December 2015 14:42:51 Hannikainen, Jaakko wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> We're developing Miracast (HDMI over Wireless connections). The
>>>>>>> current progress is that it 'works' in the userspace but doesn't have
>>>>>>> any integration with X/Wayland and can only mirror the current desktop
>>>>>>> using gstreamer.
>>>>>>>
>>>>>>> We're looking into extending the implementation so that we would be
>>>>>>> able to use the remote screens just as any other connected screen, but
>>>>>>> we're not quite sure where we should implement it.
>>>>>>>
>>>>>>> The DRM interface seems like the perfect fit since we wouldn't need to
>>>>>>> patch every compositor.
>>>>>>>
>>>>>>> Right now, gstreamer is the equivalent of the crtc/encoder, in the DRM
>>>>>>> model. Screens / crtcs are discovered using a WiFi's p2p protocol
>>>>>>> which means that screens should be hotpluggable. Since we cannot
>>>>>>> change the number of crtcs of a driver on the fly, we propose adding
>>>>>>> and removing gpus with one crtc attached and no rendering
>>>>>>> capabilities.
>>>>>>>
>>>>>>> Compositors and X currently use udev to list gpus and get run-time
>>>>>>> events for gpu hot-plugging (see the work from Dave Airlie for USB
>>>>>>> GPUs, using the modesetting X driver). We did not find a way to tell
>>>>>>> udev that we have a new device and it seems like the only way to get
>>>>>>> it to pick up our driver is from a uevent which can only be generated
>>>>>>> from the kernel.
>>>>>>>
>>>>>>> Since we have so many userspace components, it doesn't make sense to
>>>>>>> implement the entire driver in the kernel.
>>>>>>>
>>>>>>> We would thus need to have a communication from the kernel space to
>>>>>>> the userspace at least to send the flip commands to the fake crtc.
>>>>>>> Since we need this, why not implement everything in the userspace and
>>>>>>> just redirect the ioctls to the userspace driver?
>>>>>>>
>>>>>>> This is exactly what fuse / cuse [1] does, with the minor catch that
>>>>>>> it creates devices in /sys/class/cuse instead of drm. This prevents
>>>>>>> the wayland compositors and X to pick it up as a normal drm driver...
>>>>>>>
>>>>>>> We would thus need to have the drm subsystem create the device nodes
>>>>>>> for us when the userspace needs to create a new gpu. We could create a
>>>>>>> node named /dev/dri/cuse_card that, when opened, would allocate a node
>>>>>>> (/dev/dri/cardX) and would use cuse/fuse to redirect the ioctls to the
>>>>>>> process who opened /dev/dri/cuse_card.
>>>>>>>
>>>>>>> The process would then be responsible for decoding the ioctl and
>>>>>>> implementing the drm API.
>>>>>>>
>>>>>>> Since this is a major change which would allow proprietary drivers to
>>>>>>> be implemented in the userspace and since we may have missed something
>>>>>>> obvious, we would like to start a discussion on this. What are your
>>>>>>> thoughts?
>>>>>> As you raise the issue, how would you prevent proprietary userspace
>>>>>> drivers to be implemented ? Anything that would allow vendors to
>>>>>> destroy the Linux graphics ecosystem would receive a big nack from me.
>>>>> AFAIK the displaylink people already have precisely such a driver -- a
>>>>> (open-source) kernel module that allows their (closed-source)
>>>>> userspace blob to present a drm node to pass through modesetting/etc
>>>>> ioctl's.
>>>> Are you talking about the drivers/gpu/drm/udl/ driver ? I might be wrong
>>>> but I'm not aware of that kernel driver requiring a closed-source
>>>> userspace blob.
>>> Nope. That driver only works for their USB2 parts. This is what I mean:
>>>
>>> https://github.com/DisplayLink/evdi
>>> http://support.displaylink.com/knowledgebase/articles/679060
>>> http://support.displaylink.com/knowledgebase/articles/615714#ubuntu
>> Right. That's out-of-tree, people are free to screw up on their own there ;-)
> Sure, but it's identical to Jaakko's proposal from what I can
> (quickly) tell. And it's an example of someone taking an interface
> like that and writing a proprietary driver on top.
>
>    -ilia
>

You are right Ilia, this is indeed what Jaakko and I had in mind, but 
they did not re-use the fuse/cuse framework to do the serialization of 
the ioctls.

Not sure what we can do against allowing proprietary drivers to use this 
feature though :s To be fair, nothing prevents any vendor to do this 
shim themselves and nvidia definitely did it, and directly called their 
closed-source driver.

Any proposition on how to handle this case? I guess we could limit that 
to screens only, no rendering. That would block any serious GPU 
manufacturer from using this code even if any sane person would never 
write a driver in the userspace...