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

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Message-id: <55264534.4020006@samsung.com>
Date:	Thu, 09 Apr 2015 11:24:04 +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

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
that. As I wrote, current USB cable names are misleading. It would be
better to have "USB-VBUS" and "USB-ID".

>> In fact it would be better to have cables named "USB-VBUS" and "USB-ID"
>> - in this convention it would be more clear.

Thanks,
Robert Baldyga

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