Implementing Miracast?

[Ilia Mirkin]

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