> 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. -- Paul -- 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
