Sunday, March 23, 2014

Why the display server doesn't matter

Display servers are the component in the display stack that seems to hog a lot of the limelight. I think this is a bit of a mistake, as it's actually probably the least important component, at least to a user.

In the modern display stack there are five main components:
  • Hardware
  • Driver
  • Display Server / Shell
  • Toolkit / Platform API
  • Applications
The hardware we have no control over. We just get to pick which hardware to buy. The driver we have more control over - drivers range from completely closed source to fully open source. There's a tug of war between the hardware manufacturers who are used to being closed (like their hardware) and the open source community which wants to be able to modify / fix the drivers.

For (too) many years we've lived with the X display server in the open source world. But now we are moving into next generation display servers (as Apple and Microsoft did many years ago). At the moment there are two new classes of contender for X replacement, Mir and a set of Wayland based compositors (e.g. weston, mutter-wayland etc).

Applications use toolkits and platform APIs to access graphical functionality. There are plenty of toolkits out there (e.g. GTK+, Qt) and existing libraries are growing more broad, consistent and stable to be considered as a complete platform API (which is great for developers).

If you read the Internet you would think the most important part in this new world is the display server. But actually it's just a detail that doesn't matter that much.
  • Applications access the display server via a toolkit. All the successful toolkits support multiple backends because there's more than one OS out there today. In general you can take a GTK+ application and run it in Windows and everything just works.
  • The hardware and drivers are becoming more and more generic. Video cards used to have very specialised functionality and OpenGL used to provide only a fixed function function. Now video cards are basically massively parallel processors (see OpenCL) and OpenGL is a means of passing shaders and buffer contents.
The result of this is the display server doesn't matter much to applications because we have pretty good toolkits that already hide all this information from us. And it doesn't matter much to drivers as they're providing much the same operations to anything that uses them (i.e. buffer management and passing shaders around).

So what does matter now?
  • It does matter that we have open drivers. Because there will be different things exercising them we need to be able to fix drivers when display server B hits a bug but A doesn't. We saw this working with Mir on Android drivers. Since these drivers are only normally used by SurfaceFlinger there are odd bugs if you do things differently. Filing a bug report is no substitute to being able to read and fix the driver yourself.
  • The shell matters a lot more. We're moving on from the WIMP paradigm. We have multiple form factors now. The shell expresses what an application can do and different shells are likely to vary in what they allow.
I hope I've given some insight into the complex world of display stacks and shown we have plenty of room for innovation in the middle without causing major problems to the bits that matter to users. 
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