Unfortunately "pci=nomsi" didn't solve the issue (note that apart from the atom cpu, my hardware is all nvidia based, the motherboard is:AMCP7AION-HT)
I think that the follow may come handy for debugging purpose and/or to anybody with similar issues:
Using xt7-player (a gui/wrapper for mplayer which embed the video):
All happens (crashes and tearing) regardless of the composite extension state (i've disabled it via xorg.conf)
- I'm getting more crashes when i hit "stop" than when i do wild seeks around the stream.
- The first time i open a video it runs flawlessy
- if i open it again, the video will "probably" tears and all the desktop "may or not" reacts very slowly.
- mplayer in it's own window doesn't tears.
- restarting xorg make tearing goes away (again, only the first time i play the stream).
For what it's worth, my speculations against this issue is that somehow vdpau surface isn't freed correctly when the player exits, but maybe this could be addressed to mplayer itself (?).
Also i've to add that in my particular case, even if xorg hangs, the system is still alive, i can connect to it via ssh and try to kill xorg with -9, anyway, the only way to start it again is to reboot; even "rmmod nvidia" doesn't work because the module is in use.
switching to mplayer with vaapi patches
(mplayer -vo vaapi:gl -va <URI>) uses vaapi backend, doesn't crash (yet?), never tears as long as opengl is vsynced, and i can even run a composite managerwhich unredirect fullscreen windows (with tearing in window mode only).