Re: Implementing Miracast?

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Thursday 03 December 2015 10:42:50 Ilia Mirkin wrote:
> On Thu, Dec 3, 2015 at 10:34 AM, Laurent Pinchart
> 
> <laurent.pinchart@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.

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.

-- 
Regards,

Laurent Pinchart

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