Currently DRM only works for the first X session. When the user switches to another session by fast user switching or any other means, DRM and EXA are not available so rendering is unnacelerated and very slow. Using shadowfb as a fallback in that case would provide much better performance.
Reading the code again, it turns out that we would need a PreInit function to determine whether acceleration is available or not. So this needs some coding. The Init function is called after memory layout setup, which is influenced by ShadowFB.
I am encountering the sluggish UI problem on the second X session. I tried setting Option "AccelMethod" "shadowfb" as the default to force the performance to at least be acceptable on the second session but it was still really slow and now both sessions were slow. When I switched to "AccelMethod" "xaa", both sessions are usable. This probably is not the best solution but it is a good workaround until a fix becomes available.
Does this issue occur with the preferred ati driver (xf86-vide-ati)? If so, please move this to the Driver/Radeon component. Development of radeonhd has pretty much halted and development focus is on the ati driver. Please see http://www.x.org/wiki/radeonhd If the issue does not exist in the ati driver (or if there is no response to this message), this bug will be closed as WONTFIX unless someone contributes a patch.
Closing due to lack of response. Please reopen and move to the Driver/Radeon component if this issue persists with xf86-video-ati
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