When trying to run any GL application that renders something with r300g (e.g.
glxgears) fails with the message "radeon: command stream overflowed". This is a
regression caused by the following commit:

567394112d904096abff1d994ab952f475dfb444 is the first bad commit
commit 567394112d904096abff1d994ab952f475dfb444
Author: Marek Olšák <marek.olsak@amd.com>
Date:   Fri Jul 31 11:45:13 2015 +0200

    radeon/winsys: increase the IB size for VM

    Luckily, there is a kernel query, so use the size from that.
    It currently returns 256KB. It can be increased in the kernel.

    Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

I am running Linux 4.1.6 (+ some Gentoo patches) and libdrm 2.4.64. The X.org
server version is 1.16.4 and the DDX driver is xf86-video-ati-7.5.0.