Cards from r600 upwards require the firmware from x11-drivers/radeon-ucode for many things. On my r600/HD3200 everything worked, but DRI was off and 3D rendering went through llvmpipe, making e.g. nexuiz completely unplayable due to speed, and even civ4 was a giant pain. Bonsaikitten had much worse failures with his r700 and evergreen chips. So I would suggest adding the following ewarn to xorg-drivers, if emerged with VIDEO_CARDS="radeon": If you have a r600 or newer chip (HD2400 and higher numbers) and are using the free drivers you will need x11-drivers/radeon-ucode for some or all functionality. See http://www.x.org/wiki/RadeonFeature#Decoder_ring_for_engineering_vs_marketing_names to find out which family your chip belongs to. Alternatively more VIDEO_CARDS flags could be added to xorg-drivers to pull in radeon-ucode as a dependency, or this could be done with mesa.
This is all documented in The X Server Configuration HOWTO (http://www.gentoo.org/doc/en/xorg-config.xml). Couldn't we just point users there?
I upgraded to version =x11-drivers/xf86-video-ati-7.0.0 from version 6.x.x (which I believe was 6.14.4-r1), that had the unpleasant following effect : the X server would not start anymore. my card is Radeon HD 5670. Here is a log showing the problem: http://bpaste.net/show/59888/ (v7, did not run). Downgrading to version 6.14.6-r1 repaired that, here is a log: http://bpaste.net/show/59887/ (v6, runs fine) After some discussions on IRC (thanks to those who helped me), it turned out that since version 7, it is needed to have CONFIG_DRM_RADEON_KMS=y in the kernel config, and that indeed solved my problem (I also modularized the radeon driver ; I had it built-in previously). The suggestion is to mention this change in a postinst message. Note that in the kernel config, this configuration setting is marked as "NEW DRIVER" "don't enable this unless you know what you are doing" which is a little scary.
*** This bug has been marked as a duplicate of bug 356187 ***
