It's not, but the driver is... less than ideal, and has some very finicky/buggy overlap detection/merging behavior (among... other things :x). Something that appears to have been somewhat optimized/fixed on the Sage's kernel.
Meaning it's *extremely* pessimistic when merging updates, meaning you often get larger and slower refresh rectangles for no good reason, which is very much unlike the NXP driver, at which you can throw an incredible amount of rectangles per second without it batting an eye (... when it doesn't deadlock
).