adding a custom modeline with xrandr (dummy driver)

[Antoine Martin]

Hi,

Whenever an xpra client connects to a server I want to resize the dummy
xserver to match the client's resolution exactly.
The server is started with:
Xorg +extension RANDR +extension RENDER -config xorg-dummy.conf
I calculate a modeline using:
http://xtiming.sourceforge.net/cgi-bin/xtiming.pl
(say 1128x738 at 50) and add it:
xrandr --newmode "1128x738 at 50" 54.17 1128 1160 1360 1392 738 753 759 775
Then I add it to the "default" output:
xrandr --addmode "default" "1128x738 at 50"
Then I try to actually set it:
xrandr -s "1128x738 at 50"
And get this error message:
"Failed to change the screen configuration!"

Running the same commands against a real X11 server sort-of worked in
that it did set the resolution (I think - I got a black screen because
my LCD did not support it). Interestingly, it would not let me switch
back to the original resolution from a virtual terminal (same "Failed to
change..." error), I had to add a "sleep 5" to delay it and switch back
to the X11 vt to make it work. Could it be something similar with my
dummy instance? vt related? How do I fix it?
Or, what am I doing wrong?

I have tried doing this via the API rather than the xrandr commandline
and did not fare any better:
* XRRAllocModeInfo to allocate a new mode
* XRRCreateMode to create it with calculations similar to xtimings
* XRRAddOutputMode on the only output found using
XRRGetScreenResourcesCurrent
Strangely, the new mode is not listed by XRRSizes()
And calling XRRSetScreenSize actually crashed the whole X11 server!..

Thanks
Antoine