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

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Message-id: <55277FA2.6090700@samsung.com>
Date:	Fri, 10 Apr 2015 09:45:38 +0200
From:	Robert Baldyga <r.baldyga@...sung.com>
To:	Chanwoo Choi <cw00.choi@...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 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 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?

Thanks,
Robert Baldyga
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