With the 4.2 kernel support has been added for the VCE1 video encoding support. Booting with this kernel with a R280 ATI card (which supports this hardware encoder) there is a message about "TAHITI_vce.bin init error". The firmware file "TAHITI_vce.bin" file is missing from the x11-drivers/radeon-ucode package. The x11-drivers/radeon-ucode should be updated to include the _vce files from http://people.freedesktop.org/~agd5f/radeon_ucode/ especially http://people.freedesktop.org/~agd5f/radeon_ucode/TAHITI_vce.bin and http://people.freedesktop.org/~agd5f/radeon_ucode/BONAIRE_vce.bin.
It seems this package includes BONAIRE_vce.bin but misses TAHITI_vce.bin: $ equery f radeon-ucode | grep vce /lib/firmware/radeon/BONAIRE_vce.bin
A new snapshot was added in 7cd4634a9c10c18ea40f8979aaea01d2995f640c Thanks for the report.