On Thu, Jul 02, 2015 at 10:46:36AM -0400, Alan Stern wrote: > On Thu, 2 Jul 2015, Peter Chen wrote: > > > > If a control-IN transfer has a data stage that is shorter than wLength > > > and is a multiple of the ep0 maxpacket value, then the peripheral must > > > send a zero-length packet to indicate the end of the data stage. > > > Thus, the UDC driver must prepare a zero-length transaction (DTD). > > > > > > > If "one" is "multiple"? > > The wLength at setup packet is 64 bytes, Maximum Packet Length at > > dQH is 64 bytes, and the Total Bytes at dTD is 64 bytes too, does device > > must prepare a zero-length packet? > > No. Here is the exact rule for when a device must prepare a 0-length > packet for a control-IN data stage. > > Let wlength = le16_to_cpu(setup->wLength) and > let act_length = req->length be the actual data length. > Then an extra 0-length packet is needed if: > > act_length > 0 && act_length < wlength && > (act_length % maxpacket) == 0. > > (Note that if act_length == 0 then req already gives rise to a 0-length > packet; no additional packets are needed.) > > Here's another way to describe the same rule: > > 1. Every packet before the final packet must have size > equal to maxpacket. > > 2. If act_length < wlength then the final packet must have > size smaller than maxpacket. > > 3. If act_length == wlength and act_length > 0 then the final > packet must have size > 0. > > You can check that this is equivalent to the rule above. > > In the example you gave, act_length == wlength so a 0-length packet > isn't needed. Thanks, I think I understand it now. Maybe only the case A like below commit needs zero-length packet. usb: chipidea: udc: Disable auto ZLP generation on ep0 953c66469735aed8d2ada639a72b150f01dae605 > > > The host hardware will recognize when this happens, because the HCD > > > will set the appropriate bits in the data-stage qTD. For example, with > > > EHCI the HCD will set the Alternate Next qTD Pointer. Or with UHCI, > > > the HCD will set the SPD (Short Packet Detect) bit. > > > > > > > I see it in the code, but it is a null pointer (EHCI_LIST_END), does it > > mean one qTD (with valid Next qTD Pointer) can handle one 64 byte packet > > with or without zero-length packet well both? > > That is a tricky question, and the full answer involves the complicated > details of how EHCI works. > > Here is a simplified answer. If a transfer requires more than one qTD, > then all the qTDs but the last one will have their Alternate Next qTD > Pointers set to a special value. The last qTD doesn't need this. > > Since ehci-hcd never uses more than one qTD for a control transfer data > stage, the issue doesn't comes up. But if you look at how ehci-hcd > handles large bulk transfers, you will see what I mean. The case A I list above, it should uses more than one qTD, right? > > > Any reasons why iPhone can't handle zero-length packet well? > > When the iPhone receives the 64-byte packet, it thinks the data stage > is finished. It moves on to the status stage, where it sends a > 0-length packet to the gadget and waits for the acknowledgment. If the > gadget is still trying to send a 0-length packet, it won't acknowledge > the 0-length packet sent by the iPhone. The status stage will time out > and the control transfer will fail. > > Alan Stern > -- Best Regards, Peter Chen -- 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
