matroxfb: supports gamma on primary VGA out?

I'm trying to set gamma correction on the primary VGA output of my
Matrox G400 dual-head, using matroxfb with kernel 2.6.24. 

Using matroxset, the ioctl completes successfully, but there is no
effect.

stephen@alonzo:~$ strace matroxset -f /dev/fb0 -o 0 -c 0x980910 18 2>&1 | grep ioctl 
ioctl(3, MATROXFB_G_TVOCTRL or VIDIOC_G_CTRL, 0xbfaec7d4) = 0
ioctl(3, MATROXFB_S_TVOCTRL or VIDIOC_S_CTRL, 0xbfaec7d4) = 0

I've tried other controls (contrast, brightness etc.), with a range of
arguments, and none has any visible effect.

Looking in matroxfb_base.c, it looks like the implementation of
VIDIOC_S_CTRL is hardwired to use output 1. However, if I try with
CRTC1 duplicated to output 1 (using matroxset -f /dev/fb0 -o 1 128), it
still shows no effect. 

Is this a bug, or am I missing something? Thanks for any help.

Stephen

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