Enable DRI3

[Robert Ancell]

DRI3 is the replacement for DRI2. We should enable it.

[Robert Ancell (robert-ancell)]

I've heard that DRI3 caused bugs but I haven't found any evidence of what sorts of bugs or when it was enabled. Please note down if you know reasons it shouldn't be enabled.

[Cemil Azizoglu (cemil-azizoglu)]

Vulkan needs DRI3 support. It'd be good to have this enabled despite the caveats.

[Chris Halse Rogers (raof)]

At least as of the code shipped in Wily, DRI3 on Intel still has fairly obvious bugs.

The least concerning one is that Unity7 will sometimes fail to notice the invalidation of buffers on monitor hotplug, and so will alternate between a correct buffer and a mostly-undefined buffer (probably a Mesa bug).

The most concerning is that it appears to make resume-from-suspend with more than one monitor plugged in hang, apparently in the kernel (sysrq keys fail to work).

[shankao (shankao)]

Those bugs should be reported to mesa, kernel and unity7 (if they are already not) and maybe, added here, so we can keep track of the progress

[shankao (shankao)]

It seems to have being explicitly disabled in the driver after this bug report: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/1401784
It includes instructions on how to try back DRI3 and an example of test case to try with the latest code

[Marcos Alano (mhalano)]

Should we reconsider this? Three reasons for that:
- Since Vivid (hwen reported bugs occur) the driver code evolved a lot;
- Probably would be good for the migration to a GNOME desktop environment;
- Vulkan driver is becoming more present each day and it needs the DRI3.

[Timo Aaltonen (tjaalton)]

The default driver on intel hw is modesetting_drv.so now, which enables DRI3. If you need/want to change back to intel on your system, then it's just as easy to flip DRI3 on...