Yeah, I also assume that it used to work at some time. Didn't bother to
investigate, though.

> The code uses debugfs_create_atomic_t(), and atomic_t is signed, so
> I'd expect negative numbers to work.

In debugfs, it is all cast from unsigned long long to something. Check
simple_attr_write() where the value is obtained with kstrtoull. The code
fix would be to not use simple_attr, I think. I opted for the doc fix.