The last time I installed a graphics card was 10 years ago, so
it took a lot of research to find out how straightforward it is to
add the Radeon R7 240 to my ASRock A320M-HDV motherboard. To save
others some time, I here summarize the process I used (running

1. Enable IOMMU in BIOS (which is now called UEFI).
The version I have (P4.80) has IOMMU buried down several
levels and I had to ask ASRock tech support to help find it.
It's located at Advanced->AMD CBS->NBIO Common->NB Config.
I also enabled North Bridge SR-IOV and CPU->SVM Mode
2. Add "iommu=soft" to the grub command line.
Edit /etc/default/grub to append it (without the quotes) to
the command line GRUB_CMDLINE_LINUX. Then execute:
grub2-mkconfig -o /boot/grub2/grub.cfg
3. Edit xorg.conf to use the native Radeon driver.
In the Device section of /etc/X11/xorg.conf, I was able to
simply change the Driver line to "radeon" from the default

Steps 1 and 2 can conveniently be done before installing the
GPU and won't have a material effect on pre-GPU operation.