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

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Message-ID: <5722008A.3020902@ti.com>
Date:	Thu, 28 Apr 2016 15:22:34 +0300
From:	Roger Quadros <rogerq@...com>
To:	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>
CC:	"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 28/04/16 13:23, Jun Li wrote:
> Hi
> 
>> -----Original Message-----
>> From: Roger Quadros [mailto:rogerq@...com]
>> Sent: Thursday, April 28, 2016 5:55 PM
>> To: Jun Li <jun.li@....com>; stern@...land.harvard.edu; balbi@...nel.org;
>> gregkh@...uxfoundation.org; peter.chen@...escale.com
>> Cc: dan.j.williams@...el.com; jun.li@...escale.com;
>> mathias.nyman@...ux.intel.com; tony@...mide.com; Joao.Pinto@...opsys.com;
>> abrestic@...omium.org; r.baldyga@...sung.com; linux-usb@...r.kernel.org;
>> linux-kernel@...r.kernel.org; linux-omap@...r.kernel.org
>> Subject: Re: [PATCH v6 09/12] usb: gadget: udc: adapt to OTG core
>>
>> Hi,
>>
>> On 27/04/16 14:22, Roger Quadros wrote:
>>> On 26/04/16 03:07, Jun Li wrote:
>>>> Hi
>>>>
>>>>> -----Original Message-----
>>>>> From: Roger Quadros [mailto:rogerq@...com]
>>>>> Sent: Monday, April 25, 2016 10:04 PM
>>>>> To: Jun Li <jun.li@....com>; stern@...land.harvard.edu;
>>>>> balbi@...nel.org; gregkh@...uxfoundation.org;
>>>>> peter.chen@...escale.com
>>>>> Cc: dan.j.williams@...el.com; jun.li@...escale.com;
>>>>> mathias.nyman@...ux.intel.com; tony@...mide.com;
>>>>> Joao.Pinto@...opsys.com; abrestic@...omium.org;
>>>>> r.baldyga@...sung.com; linux-usb@...r.kernel.org;
>>>>> linux-kernel@...r.kernel.org; linux-omap@...r.kernel.org
>>>>> Subject: Re: [PATCH v6 09/12] usb: gadget: udc: adapt to OTG core
>>>>>
>>>>> Hi,
>>>>>
>>>>> On 21/04/16 09:38, 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"

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.

loc_conn is just an internal state variable and it corresponds to our gadget_start/stop() state.

As per 7.4.2.3
"loc_conn
The “local connect” (loc_conn) variable is TRUE when the local device has signaled that it is connected to
the bus. This variable is FALSE when the local device has signaled that it is disconnected from the bus"

Can you please point me in the specification if there is any place where loc_conn
is false and b_peripheral/a_peripheral is true?

cheers,
-roger