On Tue, 4 Sep 2018, Andrey Konovalov wrote: > Hi Alan, > > Another question. According to the USB spec: > > """ > The Data stage of a control transfer from an endpoint to the host is > complete when the endpoint does one of the following: > * Has transferred exactly the amount of data specified during the Setup stage > * Transfers a packet with a payload size less than wMaxPacketSize or > transfers a zero-length packet > """ > > AFAIU, this means that a device only needs to send a zero-length > packet when the last data packet happened to be equal to > wMaxPacketSize to indicate that the data stage is over. And if the total reply size is less than wLength in the Setup stage. > Gadgetfs seems > to be doing something different, it sets the req->zero flag (for the > response passed to usb_ep_queue) when value < w_length, where value is > the length of the response being submitted and w_length is the value > of the wLength field of the control request. Is there something I > misunderstand? No, you got it. But you're missing an important fact: The USB controller drivers will ignore the ->zero flag in cases where the message length isn't an exact multiple of wMaxPacketSize. This is what makes gadgetfs's behavior correct. Alan Stern
