Radeon 7500: Why is NetBSD the only OS in which Xorg just works?

[Nicolás Colla <nicolascollacamarote%gmail.com@localhost>]

Since the switch to kernel modesetting, the radeon driver has been *very* broken
in some of the older Radeon graphics cards like my 7500. On Linux, FreeBSD and
OpenBSD, after starting Xorg, the screen will inevitably start to flicker,
making the system freeze and making you unable to switch to a different tty,
leaving a hard reboot as your only option.

This will only stop if you disable hardware acceleration in a xorg .conf file.
Then, the flickering stops, but the system is so slow that it's unusable. Even
dragging an xterm window will lag, and the CPU gets to 100% while doing it.

On NetBSD, however, after starting X, everything works out of the box. Dragging
windows and scrolling is as fast as it was in the days of userland modesetting,
and you cannot appreciate any spike of CPU activity while dragging windows.
This makes me think there is some kind of 2D acceleration going on.
Interestingly enough, glxgears only gives a score of 20 FPS. Under Debian
Wheezy, with the UMS drivers, the score was close to 350 FPS.

And my question is: What makes NetBSD work where all other OSes fail? I have
noticed that the DRI modules are gone for Radeon in the X11R7 folder, and that
they are marked as "obsolete". Is DRI the problem? Or is it something else?