Date:	Tue, 14 Apr 2015 16:24:47 +0530
From:	Kishon Vijay Abraham I <kishon@...com>
To:	NeilBrown <neilb@...e.de>
CC:	NeilBrown <neil@...wn.name>, Tony Lindgren <tony@...mide.com>,
	<linux-api@...r.kernel.org>,
	GTA04 owners <gta04-owner@...delico.com>,
	<linux-kernel@...r.kernel.org>, Pavel Machek <pavel@....cz>,
	<cw00.choi@...sung.com>, <myungjoo.ham@...sung.com>
Subject: Re: [PATCH 0/5] Enhancements to twl4030 phy to support better charging
 - V2

Hi,

On Saturday 04 April 2015 05:58 AM, NeilBrown wrote:
> On Fri, 3 Apr 2015 19:08:22 +0530 Kishon Vijay Abraham I <kishon@...com>
> wrote:
>
>> +Extcon MAINTAINERS
>>
>> Hi,
>>
>> On Wednesday 01 April 2015 10:11 AM, NeilBrown wrote:
>>> On Thu, 26 Mar 2015 05:29:42 +0530 Kishon Vijay Abraham I <kishon@...com>
>>> wrote:
>>>
>>>> Hi NeilBrown,
>>>>
>>>> On Thursday 26 March 2015 02:52 AM, NeilBrown wrote:
>>>>> On Thu, 26 Mar 2015 02:46:32 +0530 Kishon Vijay Abraham I <kishon@...com>
>>>>> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> On Monday 23 March 2015 04:05 AM, NeilBrown wrote:
>>>>>>> Hi Kishon,
>>>>>>>    I wonder if you could queue the following for the next merge window.
>>>>>>>    They allow the twl4030 phy to provide more information to the
>>>>>>>    twl4030 battery charger.
>>>>>>>    There are only minimal changes since the first version, particularly
>>>>>>>    documentation has been improved.
>>>>>>
>>>>>> There are quite a few things in this series which use the USB PHY library
>>>>>> interface which is kindof deprecated. We should try and use the Generic PHY
>>>>>> library for all of them. It would also be better to add features to the
>>>>>> PHY framework if the we can't achieve something with the existing PHY
>>>>>> framework.
>>>>>
>>>>> Hi,
>>>>>    are you able to more specific at all?  What is the "USB PHY library"?
>>>>> Where exactly is the "PHY framework"?
>>>>
>>>> There is a USB PHY library that exists in drivers/usb/phy/phy.c and there is
>>>> a Generic PHY framework that is present in drivers/phy/phy-core.c. twl4030
>>>> actually supports both the framework.
>>>>
>>>> In your patch whatever uses struct usb_phy uses the old USB PHY library and
>>>> whatever uses struct phy uses the generic PHY framework. (Actually your patch
>>>> does not use the PHY framework at all). We want to deprecate using the USB PHY
>>>> library and make everyone use the generic PHY framework. Adding features
>>>> to a driver using the USB PHY library will make the transition to generic PHY
>>>> framework a bit more difficult.
>>>>
>>>> Now all the features that is supported in the USB PHY library may not be
>>>> supported by the PHY framework. So we should start extending the PHY framework
>>>> instead of using the USB PHY library.
>>>>
>>>> One think I noticed in your driver is using atomic notifier chain. IMO extcon
>>>> framework should be used in twl4030 USB driver to notify the controller driver
>>>> instead of using USB PHY notifier. For all other things we have to see if it
>>>> can be added in the PHY framework.
>>>
>>> I've had a look at the code with these issues in mind, and there is one issue
>>> that I'm not sure about.
>>>
>>> In phy-twl4030-usb, the usb_phy is used to hold a reference to the
>>> 'struct otg', and for passing cable state changes to the notifier.
>>
>> right now we directly call omap_musb_mailbox no? we don't use notifiers right?
>
> Correct, and omap_musb_set_mailbox uses the notifier chain.
> phy-twl4030-usb does
> 	ATOMIC_INIT_NOTIFIER_HEAD(&twl->phy.notifier);
> That is the only place the current phy code interacts with the notifier chain.

ah.. okay.
>
>
>>>
>>> The former probably has to stay until musb can keep a reference to the otg,
>>> separate form the usb_phy.  The latter can be changed to use extcon - to
>>> some extent.  I actually have patches to do that from a couple of years back,
>>> but I never proceeded with them.
>>>
>>> The problem is that one thing that needs to be communicated to the charger is
>>> the max current that was negotiated by a "Standard Downstream Port".
>>> This could be 500mA from a powered hum, or much less from an unpowered hub.
>>> (Currently the usb gadget code does negotiated between different
>>> possibilities, but it could and hopefully will one day).
>>>
>>> With the notifier chain there is an easy way to communicate the allowed
>>> current once it is negotiated. e.g. ab8500_usb_set_power() does this.
>>>
>>> 'struct phy' has no equivalent of the 'set_power' callback  which 'struct
>>> usb_phy' provides, and extcon has no mechanism (that I can see) for
>>> communicating a number - just binary cable states.
>>
>> Chanwoo Choi, Can this be modified so that we can communicate numbers like in
>> the case of EXTCON_CHARGE_DOWNSTREAM?
>>>
>>> Presumably a 'set_power' method could be added to 'struct phy' so the
>>> usb-core can communicate the number to the phy, but it is not clear to me how
>>> the 'phy' can communicate it to the charger.
>>
>> Should the PHY be involved in all this? We can make the gadget driver
>> directly communicate the value to the charger no?
>>> The 'phy' could provide an API to request the current negotiated max current,
>>> but there still needs to be a way to let the charger know that this has
>>> changed.
>>> That could in theory be done via extcon, by having a secondary
>>> 'USB_connected' cable type, but it isn't really a cable type and pretending
>>> that it is seems wrong.
>>
>> I think EXTCON_CHARGE_DOWNSTREAM was created for that purpose. Chanwoo?
>>
>
> EXTCON_CHARGE_DOWNSTREAM is something quite different.
>
> There are roughly three ways that the USB gadget can determine what sort of
> thing has been plugged in to it and what current it can draw.
>
>   - it can look at the resistance between the ID pin and GROUND.  This is a
>     physical property of the cable and it makes a lot of sense of EXTCON
>     to report different cables based on different resistances.
>
>   - it can look at the voltage provided on different pins.  If it detects a
>     certain voltage on D- when it asserts a voltage on D+, it can know
>     that it is a Charging Downstream Port (EXTCON_CHARGE_DOWNSTREAM).  This
>     might be a property of the cable (shorting D- to D+ can achieve this) or
>     might be a property of the attached device.  It makes some sense for
>     EXTCON to report cable type based on this sort of information.
>
>   - it can wait for the connected host to initiate a USB session and select a
>     particular profile.  That profile will include a "MaxPower" field.  When
>     the host selects that profile, the gadget knows it is allowed to draw that
>     much power ("current" really, measured in mA).

Thanks for that explanation :-)
>
> So EXTCON_CHARGE_DOWNSTREAM fits into the second category.  My question is
> about the third category.
> I need this "MaxPower" number to be communicated from the USB core to the
> charger driver, presumably via the "phy" driver.
>
> With "usb_phy", there is a ->set_power() callback to communicate from
> usb-core to phy, and a notifier chain to communicate from phy to charger.
> With "phy" there is nothing.

set_power sounds very specific to USB. Just thinking if we should make use of 
the regulator framework to set the current. With this the usb should create a 
dummy regulator and set the current and the charger can use the regulator.

Not sure if that makes sense though:-/

Thanks
Kishon
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