linux-kernel - Re: [PATCH v3 2/6] extcon: Add the support for extcon property according to extcon type

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Date:	Thu, 04 Aug 2016 19:57:22 +0900
From:	Chanwoo Choi <cw00.choi@...sung.com>
To:	Roger Quadros <rogerq@...com>, Guenter Roeck <groeck@...gle.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Felipe Balbi <balbi@...nel.org>, myungjoo.ham@...sung.com,
	Chris Zhong <zyw@...k-chips.com>,
	Guenter Roeck <groeck@...omium.org>, chanwoo@...nel.org,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	"cpgs (cpgs@...sung.com)" <cpgs@...sung.com>
Subject: Re: [PATCH v3 2/6] extcon: Add the support for extcon property
 according to extcon type

Hi Guenter and Roger,

On 2016년 08월 04일 17:49, Roger Quadros wrote:
> On 04/08/16 07:09, Guenter Roeck wrote:
>> On Wed, Aug 3, 2016 at 5:42 PM, Chanwoo Choi <cw00.choi@...sung.com> wrote:
>>> Hi Roger,
>>>
>>> On 2016년 08월 03일 18:46, Roger Quadros wrote:
>>>> Hi Chanwoo,
>>>>
>>
>> [ ... ]
>>
>>>>>>> +  /*
>>>>>>> +   * Check whether the external connector is attached.
>>>>>>> +   * If external connector is detached, the user can not
>>>>>>> +   * get the property value.
>>>>>>> +   */
>>>>>>
>>>>>> How will this work for USB case? We need to know VBUS and ID states
>>>>>> even if the USB cable is detached.
>>>>>
>>>>> When USB is detached, extcon_get_property return the default value without any operation.
>>>>> The default value of supported property are 0 (zero). If new property need the differnt default
>>>>> value, I'll support it.
>>>>
>>>> Is the property a property of the connector or of the cable?
>>>>
>>>> In my opinion, ID and VBUS are properties of the USB connector and not of
>>>> the USB cable. So extcon must provide valid status for those properties
>>>> even if USB cable or USB_HOST cable is detached.
>>>
>>> I don't understand about that if USB and USB_HOST are detached,
>>> how can the USB be operating? As you mentioned that, extcon must
>>> provide the valid status for both state and properties.
>>>
>>
>> Correct. No cable means that the polarity is unknown, and VBUS must not be
>> active (for USB_HOST), or it can not be active (for USB).
> 
> OK.
> 
>>
>> Only question might be EXTCON_PROP_USB_ID; I am not sure I understand
>> what it is supposed to return. Maybe it would be worthwhile to document it ?
> 
> Agreed. It seems redundant as ID can be easily inferred from USB_HOST cable state.
> i.e. if USB_HOST is attached ID is 0. If USB_HOST is detached ID is 1.

Do you mean the EXTCON_PROP_USB_ID is un-needed?
I'll remove the EXTCON_PROP_USB_ID property on next version.

> 
>>
>>> So, I already mentioned, When USB and USB_HOST are detached,
>>> extcon return the default value instead of error value.
>>> I think that it is reasonable. Why is it not a valid?
>>>
>>
>> I agree; I don't know what else could be returned if no cable is
>> attached, even if we wanted to.
>>
> OK.
> 
> I understood now that if kernel USB driver can interpret EXTCON_USB, EXTCON_USB_HOST
> and VBUS property, it sufficiently captures ID and VBUS information.
> 

Regards,
Chanwoo Choi