linux-kernel - Re: [PATCH v11 08/14] usb: otg: add OTG/dual-role core

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Date:	Mon, 20 Jun 2016 15:08:15 +0300
From:	Felipe Balbi <balbi@...nel.org>
To:	Peter Chen <hzpeterchen@...il.com>
Cc:	Roger Quadros <rogerq@...com>, peter.chen@...escale.com,
	tony@...mide.com, gregkh@...uxfoundation.org,
	dan.j.williams@...el.com, mathias.nyman@...ux.intel.com,
	Joao.Pinto@...opsys.com, sergei.shtylyov@...entembedded.com,
	jun.li@...escale.com, grygorii.strashko@...com,
	yoshihiro.shimoda.uh@...esas.com, robh@...nel.org, nsekhar@...com,
	b-liu@...com, joe@...ches.com, linux-usb@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org
Subject: Re: [PATCH v11 08/14] usb: otg: add OTG/dual-role core


Hi,

Peter Chen <hzpeterchen@...il.com> writes:
>> Roger Quadros <rogerq@...com> writes:
>> > It provides APIs for the following tasks
>> >
>> > - Registering an OTG/dual-role capable controller
>> > - Registering Host and Gadget controllers to OTG core
>> > - Providing inputs to and kicking the OTG state machine
>> 
>> I think I have already mentioned this, but after over 10 years of OTG,
>> nobody seems to care about it, why are we still touching at all I don't
>> know. For common non-OTG role-swapping we really don't need any of this
>> and, quite frankly, I fail to see enough users for this.
>> 
>> Apparently there's only chipidea which, AFAICT, already had working
>> dual-role before this OTG State Machine was added to the kernel.
>
> Some users would like to know if vendor's platform is OTG compliance,
> so we add it to pass usb.org USB OTG certification test.

I strongly doubt that's really what they mean. IMHO, users want to know
if they can swap roles. Ask them if they are really going for OTG
certification. Ask them if they have an OPT tester. Ask them if they
really want all those timers. If they want HNP polling, etc etc etc.

So far, I haven't seen anybody talking about real USB OTG (the spec)
when they say OTG. Usually they just mean "a method for swapping between
host and peripheral roles, but we really don't want all the extra cost
of the OTG specification".

> For the real use case, some Carplay platforms need it.

Carplay does *NOT* rely on OTG. Apple has its own proprietary and closed
specification which is not OTG-compliant.

>> > diff --git a/include/linux/usb/gadget.h b/include/linux/usb/gadget.h
>> > index f4fc0aa..1d74fb8 100644
>> > --- a/include/linux/usb/gadget.h
>> > +++ b/include/linux/usb/gadget.h
>> > @@ -328,6 +328,7 @@ struct usb_gadget_ops {
>> >   * @in_epnum: last used in ep number
>> >   * @mA: last set mA value
>> >   * @otg_caps: OTG capabilities of this gadget.
>> > + * @otg_dev: OTG controller device, if needs to be used with OTG core.
>> 
>> do you really know of any platform which has a separate OTG controller?
>> 
>
> It may not be a real separate OTG controller. It can be a hardware part
> (external connector, external IC, SoC OTG register area, etc) to handle vbus
> ,id and other signals which are used for role swap.

That's already solved. EXTCON solved that years back and OMAP has been
using EXTCON to program its UTMI mailbox.

-- 
balbi

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