On Sat, 1 Nov 2014, Breton M. Saunders wrote: > > This is required behavior, for example, during the data phase of a > > bulk-only mass-storage transfer. The specification states that under > > certain circumstances, the device _must_ send a zero-length packet on > > its bulk-IN endpoint. > > Hi Alan - thanks for that. Can I ask: have you got a reference for > that? It will help when I arrive back in the office on Monday. > I've briefly skimmed the usb mass storage bulk only document (from 1999) > and am looking at usb-msc-0.2.pdf now; but am not finding details on > zero-length transfers on bulk. So on careful investigation, it turns out I misspoke. In the 1.0 revision of the Bulk-Only Transport specification (the filename is probably something like usbmassbulk_10.pdf), the discussion of case 4 in section 6.7.2 says: If the device actually transfers less data than the host indicated, then: The device may end the transfer with a short packet. The device shall STALL the Bulk-In pipe. Thus, if the device chooses to end the transfer with a short packet, the packet may need to have length 0 (for case 4). But the device isn't required to do this, whereas it _is_ required to STALL the bulk-IN pipe. Ironically, many devices in the real world _do_ send a short packet and _don't_ send a STALL. Regardless, the principle is clear: Devices _should_ be able to send 0-length packets on bulk endpoints. See the discussion of short packets in sections 5.3.2 and 5.8.3 of the USB-2.0 spec. > I admit to being unfamiliar with usb mass storage; my work has primarily > been on HID and custom bulk protocols. > > I was coming at this from the user side of the problem where what I was > (incorrectly) doing was: > write(fd, buffer, 0); > where buffer is a valid virtual address, and fd is an open file > descriptor on a bulk-in endpoint. > > If I read man 2 write (ubuntu 14.04) I find that the return code for > write is undefined for a non-regular file _and_ a count value of zero. > At present I've seen fit to just return -EIO (from gadgetfs) on writes > of zero bytes to a non-control endpoint - however, I'm still trying to > ascertain whether this behavior is correct or not. gadgetfs is a whole separate story. Nevertheless, I'm pretty sure that it's supposed to return 0 for such a write, unless something goes wrong. > A document stating that zero length bulk transfers could allow me to get > permission to investigate the problem further down the chain - into musb. If there is a problem, it undoubtedly lies in the musb driver. There has been a fair amount of discussion on the mailing list recently about about how that driver should handle 0-length packets (although the discussion was concerned with control transfers, not bulk). Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
