linux-kernel - Re: [PATCH v2] extcon: otg_gpio: add driver for USB OTG port controlled by GPIO(s)

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Date:	Fri, 20 Feb 2015 11:17:00 -0800
From:	David Cohen <david.a.cohen@...ux.intel.com>
To:	Linus Walleij <linus.walleij@...aro.org>,
	Robert Baldyga <r.baldyga@...sung.com>,
	heikki.krogerus@...ux.intel.com
Cc:	MyungJoo Ham <myungjoo.ham@...sung.com>,
	Chanwoo Choi <cw00.choi@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	baolu.lu@...ux.intel.com, balbi@...com
Subject: Re: [PATCH v2] extcon: otg_gpio: add driver for USB OTG port
 controlled by GPIO(s)

Hi Linus and Robert,

CC'ing Heikki as it involves a RFC from him.

On Fri, Feb 20, 2015 at 10:53:44AM +0100, Linus Walleij wrote:
> On Fri, Feb 20, 2015 at 7:41 AM, Robert Baldyga <r.baldyga@...sung.com> wrote:
> > Hi David,
> >
> > On 02/19/2015 08:59 PM, David Cohen wrote:
> >> Some Intel platforms have an USB OTG port fully (or partially)
> >> controlled by GPIOs:
> >>
> >> (1) USB ID is connected directly to a pulled up GPIO.
> >>
> >> Optionally:
> >> (2) VBUS is enabled/disabled by a GPIO
> >> (3) Platform has 2 USB controllers connected to same port: one for
> >>     device and one for host role. D+/- are switched between phys.
> >>     according to this GPIO level.
> >>
> >> This driver configures USB OTG port for device or host role according to
> >> USB ID value.
> >>  - If USB ID's GPIO level is low, OTG port is configured for host role
> >>    by sourcing VBUS and switching D+/- to host phy.
> >>  - If USB ID's GPIO level is high, by standard, the OTG port is
> >>    configured for device role by not sourcing VBUS and switching D+/- to
> >>    device controller.
> >
> > IMO it's not very elegant to handle VBUS power on/off in extcon driver.
> > Creating fixed regulator would allow to make VBUS handling more generic.

I agree. But please, see below.

> 
> IMHO it's just layers of abstraction piled on top of each other here.
> 
> I would put this adjacent to the phy driver somewhere in drivers/usb/*
> and make the actual USB-driver thing handle its GPIOs directly.
> But I guess David and Felipe have already discussed that as we're
> seeing this patch?

Felipe suggested to "divide to conquer" instead of having a single
extcon driver to handle all these functions:

- The mux functions would be controlled by a possible new pinctrl-gpio
driver (Linus, your input here would be nice :)
- The VBUS would be a fixed regulator
- The USB ID would make usage of existent extcon-gpio

But the on fw side, this is a single ACPI device representing a virtual
device for USB OTG port, which is nothing but a bunch of independent
GPIOs.

I could make a mfd driver to register devices for those simpler and more
generic drivers, but according to [1] community recognized it as a hack
with ACPI since I'd need to give them the GPIO without requesting on
mfd.

I'm open for suggestions :)

Br, David

[1] https://lkml.org/lkml/2014/12/18/82

> 
> Yours,
> Linus Walleij
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