When I try to use DGA2, I get "resolution not supported", but the X server does actually support that resolution !

Description

When I try to use DGA2, I get "resolution not supported", but the X server does actually support that resolution !
There are two possible reasons: 1) DGA2 may support different resolutions/color depths than X, run the gfxinfo program to know what combinations you can use, 2) You may have a buggy DGA2 implementation, see next question.