Date:	Wed, 15 Apr 2015 11:27:35 +0800
From:	Peter Chen <peter.chen@...escale.com>
To:	Chanwoo Choi <cw00.choi@...sung.com>
CC:	Roger Quadros <rogerq@...com>,
	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>, "Balbi, Felipe" <balbi@...com>,
	"ABRAHAM, KISHON VIJAY" <kishon@...com>
Subject: Re: [PATCH v3 2/4] extcon: usb-gpio: add support for VBUS detection

On Tue, Apr 14, 2015 at 08:29:34PM +0900, Chanwoo Choi wrote:
> On 04/14/2015 07:38 PM, Roger Quadros wrote:
> > On 14/04/15 13:31, Chanwoo Choi wrote:
> >> On 04/14/2015 07:02 PM, Roger Quadros wrote:
> >>> Fixed Kishon's id.
> >>>
> >>> On 14/04/15 13:01, Roger Quadros wrote:
> >>>> On 10/04/15 12:18, Chanwoo Choi wrote:
> >>>>> On 04/10/2015 05:46 PM, Robert Baldyga wrote:
> >>>>>> On 04/10/2015 10:10 AM, Chanwoo Choi wrote:
> >>>>>>> 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.
> >>>>>>
> >>>>>> That's why I finally proposed to use "USB-ID" and "USB-VBUS" in parallel
> >>>>>> with old names. It seems to be simpler solution than adding new
> >>>>>> mechanism notifying about VBUS and ID states changes.
> >>>>>
> >>>>>
> >>>>> As I commented on previous reply, I don't agree to use 'USB-ID' and 'USB-VBUS'.
> >>>>> If we add new strange 'USB-ID' and 'USB-VBUS' name, we would add non-general cable
> >>>>> name continuoulsy.
> >>>>>
> >>>>> I think that extcon core provide the helper API to get the value of VBUS.
> >>>>> But I need to consider it.
> >>>>
> >>>> Now it is starting to look like existing extcon states are not suitable for USB/PHY drivers to deliver
> >>>> VBUS and ID information reliably.
> >>>> This is because based on your comments the "USB" and "USB-HOST" states look like some fuzzy states
> >>>> and have no direct correspondence with "VBUS" and "ID". The fact that they can't become
> >>>> attached simultaneously makes me conclude that "USB" and "USB-HOST" cable states are really 
> >>>> capturing only the ID pin state.
> >>>>
> >>>> I can suggest the following options
> >>>> a) let "USB" and "USB-HOST" only indicate ID pin status. Add a new cable state for "VBUS" notification.
> >>>> Maybe call it "USB-POWER" or something.
> >>
> >> We must discuss it before using the new cable name
> >> such as "USB-POWER", "USB-ID" and "USB-VBUS".
> > 
> > I didn't say to add "USB-ID" or "USB-VBUS". solution (a) was to have the following
> 
> Right. Robert suggested the "USB-ID" and "USB-VBUS" cable name on previous mail in mail thread.

>From USB/USB-PHY driver point, it needs to know id and vbus value
for their internal logic, so as extcon users, the cable name
is better to reflect meaning of id and vbus, like "USB-ID" and "USB-VBUS",
if the power is from vbus pin at USB cable, I don't think we need another
cable name "USB-POWER" even the USB/USB-PHY driver don't need it.

> 
> > "USB" - attached means ID is high. i.e. Type-B plug attached.
> > "USB-HOST" - attached means ID is low. i.e. Type-A plug attached.
> > "USB-POWER" - attached means USB power is present. i.e. VBUS is alive.
> > 
> > This way the definition of USB and USB-HOST remain true to their name and avoid further confusions.
> > VBUS state is got through the "USB-POWER" cable state.
> 
> There is the same case for MHL cable.
> Also, MHL cable could be connected to VBUS line.
> - MHL : attached just MHL cable.
> - MHL-POWER : attache MHL cable which is connected with VBUS line.
> 
> We must need the opinion of USB/PHY driver's maintainer.
> 
> > 
> >>
> >> What is the appropriate method of following two solution?
> >> - Fisrt, use the new cable name "USB-*".
> > I explained this above.
> > 
> >> - Second, use the additional API to get the VBUS state.
> > 
> > You keep mentioning additional API for VBUS. But I don't see any such API. Can you please
> > suggest what API you are talking about?
> 
> I'm considering following functions for VBUS state. But it is my opinion,
> If USB/PHY drivers's maintainers don't agree the new cable ("USB-POWER"),
> We could use the following function to get VBUS state.
> Because new cable name will affect the USB/PHY drivers.
> - int extcon_{get|set}_vbus_state(struct extcon_dev *edev);
> 
> Cheers,
> Chanwoo Choi

-- 

Best Regards,
Peter Chen
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