On Mon, 3 Nov 2014, Breton M. Saunders wrote: > Hi Alan, > > Thanks again for your help. > > I think my understanding of the bulk transfer protocol was > incorrectly confused with my previous work on control transfers. It > looks like there is no concept of "transfer" vs "transaction" in bulk > like there is in control.
There _is_ a concept of bulk transfers, but it exists only at a higher level of abstraction, whereas control transfers are represented directly within the USB bus protocol. > In bulk, the state machine consists of a > single token->data->handshake transition - the only way a "write" system > call is terminated is by writing a short packet. I'm not entirely sure what you mean here, but you seem to have the right basic idea. A host and a device agree (somehow) on how long a bulk transfer ought to be, and if the transfer needs to be shorter than the agreed-on length then it must be terminated by a short packet. Which will have to be a zero-length packet in the case where the actual length is a multiple of the maxpacket size. > Therefore, a short > packet being 0 bytes is perfectly legitimate - and therefore, performing > a write from userland of zero bytes should also be permitted. Yes. > I reckon its off I go debugging MUSB. (Note: I'm stuck using 2.6.37; > so the bug I've encountered may not be occurring on newer kernels). Good luck. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
