Missing documentation :-)
Ack.
+static inline int drm_fixed_16_16(s32 mult, s32 div)
You should return a s32.
Ack.
The function name isn't very explicit, and departs from the naming scheme of other functions in the same file. As fixed-point numbers are stored in a s64 for the drm_fixp_* helpers, we shouldn't rese the drm_fixp_ prefix, maybe drm_fixp_s16_16_ would be a good prefix. The function should probably be named drm_fixp_s16_16 from_fraction() then, but then the same logic should possibly be replicated to ensure optimal precision. I wonder if it wouldn't be best to simply use drm_fixp_from_fraction() and shift the result right by 16 bits.
Sure, I'm not attached to the naming ... will wait to hear what colours everyone else wants the bikehed painted.
As for the implementation, I just went with what was used across multiple drivers already (no chance of regressions that way) but could reuse other helpers if it's better..? If the behaviour changes this goes from a trivial cleanup to a much more invasive changeset. I don't own half of the hardware here.