https://bugs.freedesktop.org/show_bug.cgi?id=77142
--- Comment #2 from eric e.durmin@gmx.net --- The problem I've described in the top comment, is when using agpmode=4.
I somehow can not use agpmode=8. When agpmode is set to 8, the X server will not start. Loading the radeon module with modprobe is no problem, but starting the X server, for example with startx, will hang the computer. Xstart should open 3 xterm windows, but during the opening of the first xterm window the computer hanged.
When setting agpmode to 1 or 2, I get this message: [drm:radeon_agp_init] *ERROR* Illegal AGP Mode: 1 (valid 4, 8), leaving at 8
So I can only set agpmode to -1.
With agpmode=-1 the garbled text has disappeared. But certain movements on the screen can not keep up with the mouse movement. For example when moving around a window with the mouse (I use KDE's wobbling window) and scrolling a long webpage by dragging the vertical slider up and down with the mouse in Konqueror/Rekonq (Firefox doesn't have this problem).
The movement on the screen is smooth, but it is more behind the mouse than when agpmode is set to 4.
Setting agpmode to -1 is a better solution than disabeling EXAPixmaps or RenderAccel.