Re: [PATCH 0/5] Enhancements to twl4030 phy to support better charging - V2

[NeilBrown]

On Fri, 3 Apr 2015 19:08:22 +0530 Kishon Vijay Abraham I <kishon@xxxxxx>
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@xxxxxx>
> > 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@xxxxxx>
> >>> 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.


> >
> > 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).

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.

extcon _could_ be used to communicate the MaxPower, but I'm not sure it makes
sense.  It is a property of the power available on the cable, not a property
of the cable itself, or something that can be deduced from the properties of
the cable.

I guess I could get musb_gadget_vbus_draw(), or usb_phy_set_power(), to call
the notifier chain directly.  But if usb_phy is being deprecated, that isn't
a long-term solution.

Thanks,
NeilBrown

Attachment: pgp5do_6meBwe.pgp
Description: OpenPGP digital signature