Comment 44 for bug 790183

> there's no synchronization points for dev_open, so you can't use async
> transfers in it, and synchronous transfers are not allowed in libfprint.

But it works for me with async transfers in dev_open :-/ Both fprint_demo that uses asynchronous API and command-line examples using synchronous API work fine.

You gave a theoretical explanation why it's bad, but I'm not a computer programmer, I'm a physicist, and I don't understand this theory. Can you please also give a practical explanation, i.e. what exactly will break and in which situation?

As a side note, vfs301 driver, which just happened to be the one I used as an example, has synchronous transfers :p (and even separate debug system).