[Mesa-dev] Alternative graphics system based on parent/child processes

[Joseph Bates]

I've been mulling over this idea for an alternative graphics system to our current server/client models. Instead of applications connecting to a central display server, what if all graphics and input device routing were handled through the parent/child process hierarchy? For instance, your desktop environment could be towards the top of the hierarchy, and it would be responsible for launching applications as child processes. The child would then ask its parent for a graphics buffer using one of the many parent/child IPC mechanisms UNIX has to offer, and the parent would create one and hand it back. The child could then draw whatever it wants on its buffer, and the parent could composite it alongside other applications however it sees fit. Parents would also be possible for routing and transforming user input.
One benefit, for example, is how it would simplify desktop environment development. One desktop could easily be nested inside another one's window.
Where I think this becomes interesting, though, is if you think of an application like Steam. While Steam is currently a desktop application, its "Big Picture Mode" could conceivably replace a desktop environment entirely. This parent/child model would allow Steam to determine weather or not it needs to act like an application or a desktop replacement at runtime. In application mode, it could simply pass buffer requests along to its parent. But if it were in desktop replacement mode, it could grant those buffer requests itself (perhaps providing fullscreen windows only, though). In either case, Steam would have access to its child processes' graphics buffers and could render the Steam Overlay over them.
Perhaps there could even be a parent above the desktop environment process which owns all graphics and input devices. It could communicate the requests to the kernel so desktop environments just need to know the IPC protocol. Perhaps the top-level parent could also be configured to handle multiple desktop environments in-parallel with different devices, simplifying multi-seat support.
All-in-all, this approach seems more UNIXy to me. It's inspired by how command-line shells and applications handle stdio.
The reason I'm bringing it up on mesa-dev is because I'm wondering if it's even possible or reasonable with the current Linux graphics infrastructure. As I'm sure some of you are aware, the relevant documentation is scattered and I'm not always sure which ones are up-to-date and which ones aren't. I've been able to infer a little bit from the Wayland/Weston source code, but I wanted your input before I continue my investigation.
The main thing this would require is the ability to securely pass graphics buffers between processes. Is this what the DRM PRIME API is for? If buffers can be encapsulated in file descriptors, then they can be passed around using UNIX domain sockets.
Any feedback would be appreciated. Thanks! 		 	   		  
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