Re: [PATCH v6 09/12] usb: gadget: udc: adapt to OTG core

[Peter Chen <hzpeterchen@gmail.com>]

On Wed, May 04, 2016 at 11:40:56AM +0300, Roger Quadros wrote:
> On 04/05/16 11:03, Jun Li wrote:
> > Hi
> > 
> >> -----Original Message-----
> >> From: Roger Quadros [mailto:rogerq@ti.com]
> >> Sent: Wednesday, May 04, 2016 2:37 PM
> >> To: Peter Chen <hzpeterchen@gmail.com>
> >> Cc: Jun Li <jun.li@nxp.com>; stern@rowland.harvard.edu; balbi@kernel.org;
> >> gregkh@linuxfoundation.org; peter.chen@freescale.com;
> >> dan.j.williams@intel.com; jun.li@freescale.com;
> >> mathias.nyman@linux.intel.com; tony@atomide.com; Joao.Pinto@synopsys.com;
> >> abrestic@chromium.org; r.baldyga@samsung.com; linux-usb@vger.kernel.org;
> >> linux-kernel@vger.kernel.org; linux-omap@vger.kernel.org
> >> Subject: Re: [PATCH v6 09/12] usb: gadget: udc: adapt to OTG core
> >>
> >> Peter,
> >>
> >> On 04/05/16 06:35, Peter Chen wrote:
> >>> 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.
> >>>>
> >>>
> >>> Just talked with Jun, he is worried if loc_conn != pullup_dp at some
> >>> situations. So, how about only calling start gadget at
> >>> usb_start_gadget,
> >>
> >> Which situations?
> > 
> > When to pull-up DP is decided by application (while vbus is on),
> > not only by driver state machine.
> 
> So when OTG state is B_PERIPHERAL or A_PERIPHERAL we still want DP pull-up
> to be disabled?
> 
> > 
> >>
> >>> and pullup_dp at drd_set_state (see below).
> >>>
> >>>
> >>> static void drd_set_state(struct otg_fsm *fsm, enum usb_otg_state
> >>> new_state) {
> >>> 	struct usb_otg *otg = container_of(fsm, struct usb_otg, fsm);
> >>>
> >>> 	if (otg->state == new_state)
> >>> 		return;
> >>>
> >>> 	fsm->state_changed = 1;
> >>> 	dev_dbg(otg->dev, "otg: set state: %s\n",
> >>> 		usb_otg_state_string(new_state));
> >>> 	switch (new_state) {
> >>> 	case OTG_STATE_B_IDLE:
> >>> +		usb_udc_vbus_handler(gadget, false);
> >>
> >> This is redundant, When we switch from PROTO_GADGET to PROTO_UNDEF we do a
> >> usb_gadget_stop(), and a usb_gadget_disconnect() is done there.
> >>
> >>> 		drd_set_protocol(fsm, PROTO_UNDEF);
> >>> 		otg_drv_vbus(otg, 0);
> >>> 		break;
> >>> 	case OTG_STATE_B_PERIPHERAL:
> >>> 		drd_set_protocol(fsm, PROTO_GADGET);
> >>> +		usb_udc_vbus_handler(gadget, true);
> >>
> >> This is redundant as well since usb_gadget_start() is doing a
> >> usb_gadget_connect().
> >>
> >>> 		otg_drv_vbus(otg, 0);
> >>> 		break;
> >>> 	......
> >>> 	};
> >>>
> >>> }
> >>>
> >>> When the OTG FSM is added to this framework, it can keep
> >>> usb_fsm->ops->loc_conn, and using the current FSM.
> >>>
> >>
> >> I have no strong opinions for or against usb_fsm->ops->loc_conn.
> >> Although strictly speaking, we shouldn't take any action based on loc_conn.
> > 
> > Strictly speaking(OTG spec), all you does is for loc_conn, but you think
> > it's start_gadget.
> > 
> >> It is just a state variable indicator.
> > 
> > Nobody check this "indicator".
> > 
> > Of cos, this is not a big deal, you can define the new API as is,
> > do udc_start() + gadget_connect() in one shot, it's up to user to
> > decide if use your usb_otg_start_gadget(), in case of udc_start()
> > followed by gadget_connect() is not wanted, user can/need do udc_start()
> > and something else before do gadget_connect.
> 
> Please don't get me wrong. I want the full OTG state machine to be able
> to use usb_otg_start/stop_gadget().
> 

Sure, but you may change above APIs a little (delete
usb_gadget_connect).

-- 

Best Regards,
Peter Chen