On Mon, 20 Oct 2014, Paul Zimmerman wrote: > > From: linux-usb-owner@xxxxxxxxxxxxxxx [mailto:linux-usb-owner@xxxxxxxxxxxxxxx] On Behalf Of David Laight > > Sent: Monday, October 20, 2014 2:48 AM > > > > From: Felipe Balbi > > > According to Section 8.5.3.2 of the USB 2.0 specification, > > > a USB device must terminate a Data Phase with either a > > > short packet or a ZLP (if the previous transfer was > > > a multiple of wMaxPacketSize). > > > > > > For reference, here's what the USB 2.0 specification, section > > > 8.5.3.2 says: > > > > > > " > > > 8.5.3.2 Variable-length Data Stage > > > > > > A control pipe may have a variable-length data phase > > > in which the host requests more data than is contained > > > in the specified data structure. When all of the data > > > structure is returned to the host, the function should > > > indicate that the Data stage is ended by returning a > > > packet that is shorter than the MaxPacketSize for the > > > pipe. If the data structure is an exact multiple of > > > wMaxPacketSize for the pipe, the function will return > > > a zero-length packet to indicate the end of the Data > > > stage. > > > " > > > > If that is the same as my understanding of the USB3 spec then the > > requirement for a ZLP isn't unconditional. > > > > In particular if the data phase isn't variable length then a ZLP > > must not be added. > > Also, in the USB 3.0 spec, the corresponding section has been modified > a bit. The last sentence has been changed to this: > > "Note that if the amount of data in the data structure that is returned > to the host *is less than the amount requested by the host* and is an > exact multiple of maximum packet size then a control endpoint shall send > a zero length DP to terminate the data stage." (emphasis mine) > > So I think you also need to take into account the wLength field of the > request, and only send the ZLP if the amount of data returned is less > than wLength. That's right, and it's true for USB-2 as well. A ZLP is needed only in cases where the host otherwise wouldn't know the transfer is over, i.e., when the transfer length is a nonzero multiple of the maxpacket size and is smaller than wLength. It's not clear what a "variable length" control transfer is supposed to be. I guess it means that sometimes the device will send back less than wLength bytes of data. 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
