Messages in this thread | | | Date | Tue, 6 Nov 2018 09:51:06 -0500 (EST) | From | Alan Stern <> | Subject | Re: [PATCH 4/6] usb: gadget: add functions to signal udc driver to delay status stage |
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On Tue, 6 Nov 2018, Felipe Balbi wrote:
> 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.
> 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.)
> > 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.
> 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.
> > 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?
> One way to satisfy what you want, with what I want is to have UDC core > implement something like below: > > int usb_ep_start_status_stage(struct usb_gadget *g) > { > return usb_ep_queue(g->ep0, &g->ep0_status_request); > } > > special function for you, usb_ep_queue() for me :-p
Sure, this is one of the options Laurent and I have discussed.
> >> (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.)
> Driver is already required to > handle stalls for several other conditions. If thehre are bugs in that > area, I'd prefer catching them.
> > 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.
Alan Stern
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