Function probe_size 

pub fn probe_size(preferred: u32, accepts: impl FnMut(u32) -> bool) -> u32

Settle on a buffer size, asking accepts what the kernel will take.

Walks the ladder downward from preferred, and never above it: a caller who asked for a small cursor does not get a large one because the driver could manage it.

When nothing is accepted the answer is preferred, floored at 64 — there has to be a buffer either way, and the first real commit will surface the kernel’s objection if there is one. That case is usually not about size at all: with no mode set yet, every TEST_ONLY fails for unrelated reasons.