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

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Date:	Fri, 10 Apr 2015 16:39:37 +0900
From:	Chanwoo Choi <cw00.choi@...sung.com>
To:	Roger Quadros <rogerq@...com>
Cc:	Robert Baldyga <r.baldyga@...sung.com>, myungjoo.ham@...sung.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 Roger,

On 04/09/2015 06:59 PM, Roger Quadros wrote:
> Hi,
> 
> On 09/04/15 12:24, 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.
> 
> At least for the kernel users [1] we are treating USB-HOST as !ID and USB as VBUS.
> So it is not an issue for these kernel users if both USB and USB-HOST are attached.
> This is a valid USB state.

This case is only valid kernel users[1]. I think that we must
consider all case about extcon consumer driver and some framework.

> If we don't do so then extcon with 3 cable states is not sufficient to capture the
> entire USB scenario. (we need 4 states for 2 pins).
> 
> [1]
> - drivers/usb/phy/phy-omap-otg.c
> - drivers/usb/dwc3/dwc3-omap.c
> 

I think that extcon-usb-gpio.c send the only attached state of USB-HOST
and extcon could provider VBUS state with other solution instead of 
sending the attached state of both USB and USB-HOST cable at the same time.

Thanks,
Chanwoo Choi



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