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

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Date:	Fri, 05 Aug 2016 09:33:51 +0900
From:	Chanwoo Choi <cw00.choi@...sung.com>
To:	Guenter Roeck <groeck@...gle.com>
Cc:	Roger Quadros <rogerq@...com>,
	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,

On 2016년 08월 04일 23:47, Guenter Roeck wrote:
> On Thu, Aug 4, 2016 at 3:57 AM, Chanwoo Choi <cw00.choi@...sung.com> wrote:
>> 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.
>>
> 
> If ID reflects host vs. device state, yes, it is not needed. Or,
> alternatively, one of USB and USB_HOST would not be needed if ID is
> present. Since that is not feasible because it would modify the ABI to
> user space, dropping ID makes sense.

OK. I'll drop the EXTCON_PROP_USB_ID. Thanks.

Regards,
Chanwoo Choi

> 
> Thanks,
> Guenter
> 
>>>
>>>>
>>>>> 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
>>
> 
> 
>