[RFC] remoting KMS

[Thomas Hellstrom]

Hi!

I'm currently working on a remoting KMS backend, and now I thought it 
would be a good time to get some feedback on a very rough design:

The idea is that we want to be get enough information on the backend 
side of KMS to be able to remote the display system over VNC or 
something similar. Initially I'd plug in an open-source VNC server as 
the "reference" user-space.

The solution should be general enough to plug into any driver, including 
VKMS, either mirroring an existing crtc/connector or adding a virtual 
one. The driver would typically only have to implement an efficient 
readback command, optionally with a diff-map or damage rect that tracks 
changed content for the VNC server to be able to do its work efficiently.

This would all be forwarded to a user-space interface and protocol with 
a slightly more elaborate user-space event-mechanism than the one we 
have in drm now. In particular we want to be able to read partial 
events, and resume event reading after a partial event. We would also 
want to be able to send commands either through an ioctl() or using 
write(). Current test code is using write().

Since the VNC server in our application typically will be system-wide, 
we need to resurrect a control-node functionality. I was trying to use 
sysfs attributes for this, but while sysfs supports rudimentary poll 
functionality. It's a bit too rudimentary and tracking client lifetime 
becomes racy. Perhaps control nodes renamed to "remoting" nodes or 
something similar. It might be that masters want to attach a VNC server 
to their current display and in that case we probably would want to 
expand primary nodes with this functionality as well.

Now to the open-source  problem. As mentioned, the reference user-space 
will be a solution based on libvncserver. In fact there will be a small 
user-space interface library that libvncserver can interact with, 
similar to libdrm. However, I envision in the long run VMWare would be 
interested to use other remoting protocols with closed source protocol 
encoders, using the open source reference library. In theory, this 
solution would also allow for simple (open- or closed source) user-space 
display drivers, very similar to DisplayLink's evdi solution, even if 
that's not a use-case VMWare is currently interested in.

Thoughts and comments appreciated.

/Thomas