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

[Kishon Vijay Abraham I]

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@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.

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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