Implementing Miracast?

[Ilia Mirkin]

On Thu, Dec 3, 2015 at 10:34 AM, Laurent Pinchart
<laurent.pinchart at ideasonboard.com> wrote:
> Hi Jaakko,
>
> 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.

  -ilia