On 23 January 2017 at 19:57, Felipe Balbi <balbi@xxxxxxxxxx> wrote: > > Hi, > > Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes: >> On Mon, 16 Jan 2017, Felipe Balbi wrote: >> >>> > The gadget driver never calls usb_ep_queue in order to receive the next >>> > SETUP packet; the UDC driver takes care of SETUP handling >>> > automatically. >>> >>> yeah, that's another thing I'd like to change. Currently, we have no >>> means to either try to implement device-initiated LPM without adding a >>> ton of hacks to UDC drivers. If we require upper layers (composite.c, >>> most of the time) to usb_ep_queue() separate requests for all 3 phases >>> of a ctrl transfer, we can actually rely on the fact that a new SETUP >>> phase hasn't been queued yet to trigger U3 entry. >> >> I haven't given any thought to LPM. > > okay > >> However, requiring gadget drivers to request SETUP packets seems rather >> questionable. It flies against the USB spec, which requires > > right, maybe SETUP is a bit of an overkill. DATA and STATUS, however, > should be doable. > >> peripherals to accept SETUP packets at any time -- a device is not >> allowed to NAK or STALL a SETUP packet (see 8.4.6.4 in the USB-2 spec). >> In fact, the hardware in UDCs probably isn't capable of doing it. >> >> This means that to do what you want, the UDC driver would have to >> accept SETUP packets at any time, and store the most recent packet >> contents. Then, when the gadget driver submits a request, the UDC >> driver would give it this stored data. It would also have to detect > > that's right, I missed that part. > >> and prevent a nasty race where the gadget driver tries to queue a >> request on ep0 that is a response to an old SETUP, one that has already >> been overwritten. I'm not even sure preventing this race would be >> possible in your scheme. >> >> The advantage to invoking the gadget driver's setup callback directly >> from the UDC driver's interrupt handler is that the gadget driver will >> know immediately when an old SETUP has become stale. (That's what >> ep0_req_tag is for in f_mass_storage.) It also provides a concurrency >> guarantee, because the driver does not re-enable UDC SETUP interrupts >> until the handler is finished. >> >>> Another detail that this helps is that PM (overall) becomes simpler as, >>> most likely, we won't need to mess with transfer cancellation, for >>> example. >> >> System PM on a gadget is always troublesome. Even if the USB >> connection is a wakeup source, it may not be possible to guarantee that >> the gadget can wake up quickly enough to handle an incoming packet. > > that's true. > >>> > You are suggesting that status stage requests should not be queued >>> > automatically by UDC drivers but instead queued explicitly by gadget >>> > drivers. This would mean changing every UDC driver and every gadget >>> > driver. >>> >>> yes, a bit of work but has been done before. One example that comes to >>> mind is when I added ->udc_start() and ->udc_stop(). It's totally >>> doable. We can, for instance, add a temporary >>> "wants_explicit_ctrl_phases" flag to struct usb_gadget which, if set, >>> will tell composite.c (or whatever) that the UDC wants explicitly queued >>> ctrl phases. >> >> The term used in the USB spec is "stage", not "phase". "Phase" refers >> to the packets making up a single transaction: token, data, and >> handshake. >> >> Also, data stages are already explicit. So your temporary flag might >> better be called "wants_explicit_status_stages". > > I stand corrected ;-) > >>> Then add support for that to each UDC and set the flag. Once all are >>> converted, add one extra patch to remove the flag and the legacy >>> code. This has, of course, the draw back of increasing complexity until >>> everything is converted over; but if it's all done in a single series, I >>> can't see any problems with that. >>> >>> > Also, it won't fix the race that Baolin Wang found. The setup routine >>> >>> well, it will help... see below. >>> >>> > is always called in interrupt context, so it can't sleep. Doing >>> > anything non-trivial will require a separate task, and it's possible >>> > that this task will try to enqueue the data-stage or status-stage >>> > request before the UDC driver is ready to handle it (for example, >>> > before or shortly after the setup routine returns). >>> > >>> > To work properly, the UDC driver must be able to accept a request for >>> > ep0 any time after it invokes the setup callback -- either before the >>> > callback returns or after. >>> >>> Right, all UDCs are *already* required to support this case anyway >>> because of USB_GADGET_DELAYED_STATUS. There was a bug in DWC3, sure, but >>> it was already required to support this case. >>> >>> By removing USB_GADGET_DELAYED_STATUS altogether and making phases more >>> explict, we enforce this requirement and it'll be much easier to test >>> for it IMO. >> >> Okay, I can see the point of requiring explicit status requests. >> Implementing it will be a little tricky, because right now some status >> requests already are explicit (those for length-0 OUT transfers) while >> others are implicit. > > exactly. And that's source of issues for every new UDC driver we get. > >> (One possible approach would be to have the setup routine return >> different values for explicit and implicit status stages -- for >> example, return 1 if it wants to submit an explicit status request. >> That wouldn't be very different from the current >> USB_GADGET_DELAYED_STATUS approach.) > > not really, no. The idea was for composite.c and/or functions to support > both methods (temporarily) and use "gadget->wants_explicit_stages" to > explicitly queue DATA and STATUS. That would mean that f_mass_storage > wouldn't have to return DELAYED_STATUS if > (gadget->wants_explicit_stages). > > After all UDCs are converted over and set wants_explicit_stages (which > should all be done in a single series), then we get rid of the flag and > the older method of DELAYED_STATUS. (Sorry for late reply due to my holiday) I also met the problem pointed by Alan, from my test, I still want to need one return value to indicate if it wants to submit an explicit status request. Think about the Control-IN with a data stage, we can not get the STATUS phase request from usb_ep_queue() call, and we need to handle this STATUS phase request in dwc3_ep0_xfernotready(). But Control-OUT will get one 0-length IN request for the status stage from usb_ep_queue(), so we need one return value from setup routine to distinguish these in dwc3_ep0_xfernotready(), or we can not handle status request correctly. Maybe I missed something else. > >> On the other hand, I am very doubtful about requiring explicit setup >> requests. > > right, me too ;-) So do you suggest me continue to try to do this? Thanks. -- Baolin.wang Best Regards -- 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
