[compiz] possible division by zero in screen.c

[Nicholas Thomas]

Hi,

In detectRefreshRateOfScreen(), we have

s->redrawTime = 1000 / s->opt[COMP_SCREEN_OPTION_REFRESH_RATE].value.i;

When the refresh rate is autodetected through RandR's XRRConfigCurrentRate 
function, 0 can be returned in certain cases, leading to division by zero. An 
example of this situation would be the DynamicTwinView option in the nVidia 
proprietary drivers[0] (note that this is the default option in TwinView 
configurations). Apparently, it can happen in other situations as well.

A simple patch that falls back to the manually-set refresh rate when a SIGFPE 
is imminent is attached, but I don't think it's really the best way to deal 
with this situation. In particular, where the index doesn't = 0, there'll be 
no crash but the refresh rate will be stupidly slow. A better solution might 
be to fall back to the manual setting where a stupid (e.g.  < 50 ?) refresh 
rate has been autodetected.

Technically, it's the driver behaving in a broken way, but it'd be nice to 
avoid the crash.

/Nick ("lupine" / "lupine_85")
 
[0]http://http.download.nvidia.com/XFree86/Linux-x86/1.0-9755/README/appendix-d.html
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