Date:	Wed, 4 May 2016 09:37:29 +0300
From:	Roger Quadros <rogerq@...com>
To:	Peter Chen <hzpeterchen@...il.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

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?

> 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.
It is just a state variable indicator.

cheers,
-roger

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