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.
> I would like to double confirm it since the iPhone (As host after HNP)
> can't work well with it if I have a zero-length packet after 64 bytes packet
> which I describe above, if without zero-length packet, the iPhone works well.
That means the iPhone is behaving properly.
> > 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.
> 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
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