Date:	Wed, 4 May 2016 09:47:05 +0800
From:	Peter Chen <hzpeterchen@...il.com>
To:	Roger Quadros <rogerq@...com>
Cc:	Jun Li <jun.li@....com>,
	"stern@...land.harvard.edu" <stern@...land.harvard.edu>,
	"balbi@...nel.org" <balbi@...nel.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"peter.chen@...escale.com" <peter.chen@...escale.com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"jun.li@...escale.com" <jun.li@...escale.com>,
	"mathias.nyman@...ux.intel.com" <mathias.nyman@...ux.intel.com>,
	"tony@...mide.com" <tony@...mide.com>,
	"Joao.Pinto@...opsys.com" <Joao.Pinto@...opsys.com>,
	"abrestic@...omium.org" <abrestic@...omium.org>,
	"r.baldyga@...sung.com" <r.baldyga@...sung.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>
Subject: Re: [PATCH v6 09/12] usb: gadget: udc: adapt to OTG core

On Tue, May 03, 2016 at 06:44:46PM +0300, Roger Quadros wrote:
> Hi,
> 
> On 03/05/16 10:06, Jun Li wrote:
> > Hi
> > 
> >>>>>>>>>  /**
> >>>>>>>>> + * usb_gadget_start - start the usb gadget controller and
> >>>>>>>>> +connect to bus
> >>>>>>>>> + * @gadget: the gadget device to start
> >>>>>>>>> + *
> >>>>>>>>> + * This is external API for use by OTG core.
> >>>>>>>>> + *
> >>>>>>>>> + * Start the usb device controller and connect to bus (enable
> >> pull).
> >>>>>>>>> + */
> >>>>>>>>> +static int usb_gadget_start(struct usb_gadget *gadget) {
> >>>>>>>>> +	int ret;
> >>>>>>>>> +	struct usb_udc *udc = NULL;
> >>>>>>>>> +
> >>>>>>>>> +	dev_dbg(&gadget->dev, "%s\n", __func__);
> >>>>>>>>> +	mutex_lock(&udc_lock);
> >>>>>>>>> +	list_for_each_entry(udc, &udc_list, list)
> >>>>>>>>> +		if (udc->gadget == gadget)
> >>>>>>>>> +			goto found;
> >>>>>>>>> +
> >>>>>>>>> +	dev_err(gadget->dev.parent, "%s: gadget not registered.\n",
> >>>>>>>>> +		__func__);
> >>>>>>>>> +	mutex_unlock(&udc_lock);
> >>>>>>>>> +	return -EINVAL;
> >>>>>>>>> +
> >>>>>>>>> +found:
> >>>>>>>>> +	ret = usb_gadget_udc_start(udc);
> >>>>>>>>> +	if (ret)
> >>>>>>>>> +		dev_err(&udc->dev, "USB Device Controller didn't
> >>>> start: %d\n",
> >>>>>>>>> +			ret);
> >>>>>>>>> +	else
> >>>>>>>>> +		usb_udc_connect_control(udc);
> >>>>>>>>
> >>>>>>>> For drd, it's fine, but for real otg, gadget connect should be
> >>>>>>>> done by
> >>>>>>>> loc_conn() instead of gadget start.
> >>>>>>>
> >>>>>>> It is upto the OTG state machine to call gadget_start() when it
> >>>>>>> needs to connect to the bus (i.e. loc_conn()). I see no point in
> >>>>>>> calling gadget start before.
> >>>>>>>
> >>>>>>> Do you see any issue in doing so?
> >>>>>>
> >>>>>> This is what OTG state machine does:
> >>>>>> case OTG_STATE_B_PERIPHERAL:
> >>>>>>      otg_chrg_vbus(otg, 0);
> >>>>>>      otg_loc_sof(otg, 0);
> >>>>>>      otg_set_protocol(fsm, PROTO_GADGET);
> >>>>>>      otg_loc_conn(otg, 1);
> >>>>>>      break;
> >>>>
> >>>> On second thoughts, after seen the OTG state machine.
> >>>> otg_set_protocol(fsm, PROTO_GADGET); is always followed by
> >>>> otg_loc_conn(otg, 1); And whenever protocol changes to anything other
> >>>> the PROTO_GADGET, we use otg_loc_conn(otg, 0);
> >>>>
> >>>> So otg_loc_conn seems redundant. Can we just get rid of it?
> >>>>
> >>>> usb_gadget_start() implies that gadget controller starts up and
> >>>> enables pull.
> >>>> usb_gadget_stop() implies that gadget controller disables pull and
> >> stops.
> >>>>
> >>>>
> >>>> Can you please explain why just these 2 APIs are not sufficient for
> >>>> full OTG?
> >>>>
> >>>> Do we want anything to happen between gadget controller start/stop
> >>>> and pull on/off?
> >>>
> >>> "loc_conn" is a standard output parameter in OTG spec, it deserves a
> >>> separate api, yes, current implementation of OTG state machine code
> >>> seems allow you to combine the 2 things into one, but don't do that,
> >>> because they do not always happen together, e.g. for peripheral only B
> >>> device (also a part OTG spec: section 7.3), will be fixed in gadget
> >>> mode, but it will do gadget connect and disconnect in its diff states,
> >>> so, to make the framework common, let's keep them separated.
> >>
> >> I'm sorry but I didn't understand your comment about "it will do gadget
> >> connect and disconnect in its diff states"
> > 
> > Gadget connect means loc_conn(1).
> > 
> >>
> >> I am reading the OTG v2.0 specification and loc_conn is always true when
> >> b_peripheral or a_peripheral is true and false otherwise.
> > 
> > If you only talk about these 2 states, yes, loc_conn is ture.
> > 
> >>
> >> loc_conn is just an internal state variable and it corresponds to our
> >> gadget_start/stop() state.
> > 
> > It's not an internal variable, there are OTG state machine
> > parameters tables(table 7-x) in OTG spec which have clear lists
> > which are "internal variable", which are "input", which are "output"...
> > 
> > Those APIs are driven directly from OTG spec, easily understood so
> > code reader can know what's those APIs for. For real OTG, I don't
> > see the benefit if get rid of it.
> 
> OK, no issues if we don't get rid of it. But I am still in favor of
> doing a connect in usb_gadget_start(), because
> 
> 1) If we split connect/disconnect() and usb_gadget_start/stop() then there is
> additional overhead of keeping track whether connect was called or not during
> usb_gadget_stop(). Plus we need to take care that users don't call connect/disconnect
> outside of start/stop. It is just complicating things.
> 
> 2) for many controllers there is no difference between run/stop and
> connect/disconnect. i.e. a single register bit controls both.
> 
> 3) it fits well with the OTG specification. OTG specification says
> that loc_conn *variable* must be true *after* the device has signalled a connect.
> So OTG state machine can safely set loc_conn variable to true after doing
> otg_set_protocol(fsm, PROTO_GADGET); and set it to false otherwise.
> 
> Note, OTG specification does not say to take any action based on loc_conn.
> It is just a connect indicator variable. So we might have to fix this in the
> OTG state machine.
> 
> My suggestion is to keep it simple for now. Try the OTG implementation,
> and later if we find issues then extend it as required.
> 

I agree with you, roger.

loc_conn action is only needed for peripheral, we can't do real !loc_conn
at host mode. Besides, loc_conn is the output, we need to set/clear it after
state has changed, and current gadget framework already takes well for
connection and disconnection state, so it is better to delete loc_conn
action at otg_fsm->ops, and only keeps it as indicator for OTG FSM
reference.

-- 

Best Regards,
Peter Chen

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