From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tony Lindgren Subject: Re: [RFC][PATCH 1/9] usb: hcd: Introduce usb_start/stop_hcd() Date: Wed, 18 Mar 2015 14:41:41 -0700 Message-ID: <20150318214140.GY31346@atomide.com> References: <1426686963-11613-2-git-send-email-rogerq@ti.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-usb-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Alan Stern Cc: Roger Quadros , gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org, balbi-l0cyMroinI0@public.gmane.org, dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org, peter.chen-KZfg59tc24xl57MIdRCFDg@public.gmane.org, jun.li-KZfg59tc24xl57MIdRCFDg@public.gmane.org, mathias.nyman-VuQAYsv1563Yd54FQh9/CA@public.gmane.org, linux-usb-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-omap-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-omap@vger.kernel.org * Alan Stern [150318 12:50]: > On Wed, 18 Mar 2015, Roger Quadros wrote: > > > To support OTG we want a mechanism to start and stop > > the HCD from the OTG state machine. Add usb_start_hcd() > > and usb_stop_hcd(). > > > > Signed-off-by: Roger Quadros > > There are a few problems in this proposed patch. > > > +int usb_start_hcd(struct usb_hcd *hcd) > > +{ > > + int retval; > > + struct usb_device *rhdev = hcd->self.root_hub; > > + > > + if (hcd->state != HC_STATE_HALT) { > > + dev_err(hcd->self.controller, "not starting a running HCD\n"); > > + return -EINVAL; > > + } > > + > > + hcd->state = HC_STATE_RUNNING; > > + retval = hcd->driver->start(hcd); > > + if (retval < 0) { > > + dev_err(hcd->self.controller, "startup error %d\n", retval); > > + hcd->state = HC_STATE_HALT; > > + return retval; > > + } > > + > > + /* starting here, usbcore will pay attention to this root hub */ > > + if ((retval = register_root_hub(hcd)) != 0) > > + goto err_register_root_hub; > > If the host controller is started more than once, you will end up > unregistering and re-registering the root hub. The device core does > not allow this. Once a device has been unregistered, you must not try > to register it again -- you have to allocate a new device and register > it instead. > > Also, although you call the driver's ->start method multiple times, the > ->reset method is called only once, when the controller is first > probed. It's not clear that this will work in a situation where the HC > and the UDC share hardware state; after the UDC is stopped it may be > necessary to reset the HC before it can run again. > > It might be possible to make this work, but I suspect quite a few > drivers would need rewriting first. As another example of the problems > you face, consider how stopping a host controller will interact with > the driver's PM support (both system suspend and runtime suspend). > > It would be a lot simpler to unbind the host controller driver > completely when switching to device mode and rebind it when switching > back. I guess that is the sort of heavy-duty approach you want to > avoid, but it may be the only practical way forward. Hmm from memory I think the OTG spec assumes the USB devices are suspended when attempting the role change? I could be totally wrong, it's been a really long time since I've looked at the OTG spec, but maybe that would make it easier to deal with thing? Regards, Tony -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html