Mir
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In order to help with debugging, it's important to understand what XMir is and what bugs are likely to occur. XMir is not a replacement to X and does not alter X or compiz window management functionality in the current Ubuntu stack. XMir is an addition to the system. XMir enables Mir to be employed as a system compositor under the X stack, in this configuration Mir composites the greeter and the desktop sessions.
XMir is currently targeted at the Ubuntu 13.10 release as being the default configuration. It currently supports open source graphics drivers based on Mesa, Nouveau and Radeon. Once XMir is installed and properly configured, upon boot or restart of the LightDM daemon, Mir will determine if there is proper driver support; in the instance where proprietary drivers not supporting the Mir driver model are detected, LightDM will continue to boot into the standalone X configuration without error. From a visual perspective on Ubuntu desktop, XMir and standalone X are identical (note, currently there is an X cursor present on XMir but this soon be disabled, making the experience truly identical and indiscernible without checking on what processes are running, see Using Mir on a PC).
In order to return to a standalone X configuration, comment out the following line in /etc/lightdm/lightdm.conf.d/10-unity-system-compositor.conf to look to look like this:
[SeatDefaults] #type=unity
Simply reboot or restart LightDM. To reverse the process, simply comment the line back in to enable XMir.
At the moment, the most common bugs are related to failures to boot into XMir. Due to the robust fallback mechanisms in place, most failures will result in completing the boot sequence standalone X configuration, when you expected XMir.
If you experience a UI lock up or a crash, it would be helpful to double check for X backtracing which can be found here: https://wiki.ubuntu.com/X/Debugging
Note, it is highly unlikely you will see any visual corruption due to XMir. If you experience a visual corruption during use, it is most likely that it a bug that exists in the standalone X Ubuntu configuration. Please attempt to toggle back to the standalone X configuration and replicate the use case. If you succeed in only being able to repeat the visual corruption in XMir and not in standalone X, please file a bug with a detailed description of hardware used, XMir component versions used and the use case steps. You may check the versions of key XMir components by the following
dpkg -s libmirclient3 | grep Version dpkg -s libmirserver0 | grep Version dpkg -s lightdm | grep Version dpkg -s unity-system-compositor | grep Version
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Generated on Tue Mar 24 16:15:19 UTC 2015