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
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