I am using a Yeeloong 8133 with Loongson 3A processor (mips64el). GPU is an AMD 780E. I built linux 3.15 with the default .config for this machine: arch/mips/configs/loongson3_defconfig. When I boot with this kernel, my display does not show any information. I suspect something is wrong with the radeon video driver.
The culprit commit is https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=01ac8794a77192236a4b91c33adf4177ac5a21f0 by Alex Deucher: drm/radeon: re-order firmware loading in preparation for dpm rework.
Make sure you have the necessary ucode installed and available on your initrd if you are uing one.
Where can I get this ucode? It's obviously not part of the kernel sources, why?
Firmware is not allowed in the kernel source tree. It's included in the linux-firmware tree: http://git.kernel.org/cgit/linux/kernel/git/firmware/linux-firmware.git and most distros provide packages for it.
The radeon firmware is clearly non-free. Is there a way to make linux 3.15 boot on my computer without this firmware?
We don't support using the driver without the firmware loaded. It's requried for too much functionality. Without it, you don't have support for acceleration or interrupts.
In that case, is it possible to have graphics on an AMD/ATI GPU without the radeon driver?
You can use vesa or efifb if your platform supports it.