Re: [PATCH 4/6] usb: gadget: add functions to signal udc driver to delay status stage

[Felipe Balbi]

Hi,

Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes:
>> DATA stage always depends on a usb_ep_queue() from gadget driver. So
>> it's always "delayed" in that sense.
>
> However, it's conceivable that some UDC drivers might behave 
> differently depending on whether the usb_ep_queue call occurs within 
> the setup callback or after that callback returns.  They _shouldn't_, 
> but they might.

but now we're speculating. Should we really care before we catch
regressions?

>> it avoids all the special cases. UDC drivers can implement a single
>> handling for struct usb_request. We could do away with special return
>> values and so on...
>
> It's not quite so simple, because the UDC driver will need to keep 
> track of whether a request queued on ep0 should be in the IN or the OUT 
> direction.  (Maybe they have to do this already, I don't know.)

UDC drivers already have to do that.

>> > request and the UDC would then need to check whether that request corresponds 
>> > to a status stage and process it accordingly. A new operation specific to this 
>> 
>> no, it wouldn't. UDC would have to check the size of request, that's
>> all:
>> 
>> 	if (r->length == 0)
>>         	special_zlp_handling();
>> 	else
>>         	regular_non_zlp_handling();
>
> Checking the length isn't enough.  A data stage can have 0 length.

apologies, I meant wLength, like so:

	len = le16_to_cpu(ctrl->wLength);
	if (!len) {
		dwc->three_stage_setup = false;
		dwc->ep0_expect_in = false;
		dwc->ep0_next_event = DWC3_EP0_NRDY_STATUS;
	} else {
		dwc->three_stage_setup = true;
		dwc->ep0_expect_in = !!(ctrl->bRequestType & USB_DIR_IN);
		dwc->ep0_next_event = DWC3_EP0_NRDY_DATA;
	}

>> But we don't need to care about special return values and the like. We
>> don't even need to care (from UDC perspective) if we're dealing with
>> 2-stage or 3-stage control transfers (well, dwc3 needs to care because
>> of different TRB types that needs to be used, but that's another story)
>
> No, we do need to care because of the direction issue.

special return values would be rendered uncessary if there's agreement
that status stage is always explicit. Why would need
USB_GADGET_DELAYED_STATUS if every case returns that?

>> > There's also the fact that requests can specify a completion handler, but only 
>> > the data stage request would see its completion handler called (unless we 
>> > require UDCs to call completion requests at the completion of the status 
>> > stage, but I'm not sure that all UDCs can report the event to the driver, and 
>> > that would likely be useless as nobody needs that feature).
>> 
>> you still wanna know if the host actually processed your status
>> stage. udc-core can (and should) provide a generic status stage
>> completion function which, at a minimum, aids with some tracepoints.
>
> Helping with tracepoints is fine.  However, I don't think function 
> drivers really need to know whether the status stage was processed by 
> the host.  Can you point out any examples where such information would 
> be useful?

If you know your STATUS stage completed, you have a guarantee that your
previous control transfer is complete. It's a very clear signal that you
should prepare for more control transfers.

>> >> (But it does involve a
>> >> race in cases where the host gets tired of waiting and issues another
>> >> SETUP packet before the processing of the first transfer is finished.)
>> 
>> Host would stall first in that case.
>
> I don't follow.  Suppose the host sends a SETUP packet for an IN 
> transfer, but the gadget takes so long to send the IN data back that 
> the host times out.  So then the host sends a SETUP packet for a new 
> transfer.  No stalls.
>
> (Besides, hosts never send STALL packets anyway.  Only peripherals do.)

oh okay. This is the setup_packet_pending case.

>> > To simplify function drivers, do you think the above proposal of adding a flag 
>> > to the (data stage) request to request an automatic transition to the status 
>> > stage is a good idea ? We could even possibly invert the logic and transition 
>> 
>> no, I don't think so. Making the status phase always explicit is far
>> better. UDCs won't have to check flags, or act on magic return
>> values. It just won't do anything until a request is queued.
>
> I don't agree.  This would be a simple test in a localized area (the 
> completion callback for control requests).  It could even be 
> implemented by a library routine; the UDC driver would simply have to 
> call this routine immediately after invoking the callback.

I don't follow what you mean here.

-- 
balbi

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