linux-kernel - Re: [PATCH v3 2/4] extcon: usb-gpio: add support for VBUS detection

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Message-id: <55278580.2090809@samsung.com>
Date:	Fri, 10 Apr 2015 17:10:40 +0900
From:	Chanwoo Choi <cw00.choi@...sung.com>
To:	Robert Baldyga <r.baldyga@...sung.com>
Cc:	myungjoo.ham@...sung.com, rogerq@...com, linux-usb@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	m.szyprowski@...sung.com
Subject: Re: [PATCH v3 2/4] extcon: usb-gpio: add support for VBUS detection

On 04/10/2015 04:45 PM, Robert Baldyga wrote:
> On 04/10/2015 09:17 AM, Chanwoo Choi wrote:
>> Hi Robert,
>>
>> On 04/09/2015 06:24 PM, Robert Baldyga wrote:
>>> Hi Chanwoo,
>>>
>>> On 04/09/2015 11:07 AM, Chanwoo Choi wrote:
>>>> Hi Robert,
>>>>
>>>> On 04/09/2015 04:57 PM, Robert Baldyga wrote:
>>>>> Hi Chanwoo,
>>>>>
>>>>> On 04/09/2015 04:12 AM, Chanwoo Choi wrote:
>>>>>> Hi Robert,
>>>>>>
>>>>
>>>> [snip]
>>>>
>>>>>> But, I have one question about case[3]
>>>>>>
>>>>>> If id is low and vbus is high, this patch will update the state of both USB and USB-HOST cable as attached state.
>>>>>> Is it possible that two different cables (both USB and USB-HOST) are connected to one port simultaneously?
>>>>>>
>>>>>
>>>>> It's because state of single USB cable connection cannot be completely
>>>>> described using single extcon cable. USB cable state has two bits (VBUS
>>>>> and ID), so we need to use two cables for single cable connection. We
>>>>> use following convention:
>>>>> cable "USB" = VBUS
>>>>> cable "USB-HOST" = !ID.
>>>>
>>>> I think that extcon provider driver have to update the only one cable state
>>>> of either USB or USB-HOST because USB and USB-HOST feature can not be used
>>>> at the same time through one h/w port.
>>>>
>>>> If extcon-usb-gpio.c update two connected event of both USB and USB-HOST cable
>>>> at the same time, the extcon consumer driver can not decide what handle either USB or USB-HOST.
>>>>
>>>
>>> It can. USB OTG allows for that. Moreover device can be host even if
>>> ID=1 (so detected cable type is USB device), or peripheral when ID=0 (so
>>> detected cable type is USB host). Devices would need to have complete
>>> information about USB cable connection, because OTG state machine needs
>>
>> As I knew, USB OTG port don't send the attached cable of both USB and USB-HOST
>> at the same time. The case3 in your patch update two cable state about one h/w port.
>>
> 
> It's because simple "USB" or "USB-HOST" means nothing for USB OTG
> machine. It needs to know exact VBUS and ID states, which cannot be
> concluded basing on cable type only. That's why I have used "USB-HOST"
> name together with "USB" to pass additional information about USB cable
> connection.

I think this method is not proper to support this case.
It may cause the confusion about other case using USB/USB-HOST cable state
except of you commented case.

> 
>> I don't agree. 
>>
>>> that. As I wrote, current USB cable names are misleading. It would be
>>> better to have "USB-VBUS" and "USB-ID".
>>
>> It is strange cable name. I prefer to use only 'USB' cable name.
>> But, we could support the other method to get the state of whether USB-VBUS or USB-ID
>> by using helper API or others.
>>
> 
> Ok, so do you have any idea how to do it? Do we want to supply
> additional API for notifying about VBUS and ID changes?

No, we need to consider more standard solution to support this case.




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