Date:   Thu, 03 Nov 2016 12:25:42 +1100
From:   NeilBrown <neilb@...e.com>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Sebastian Reichel <sre@...nel.org>,
        Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
        David Woodhouse <dwmw2@...radead.org>, robh@...nel.org,
        Jun Li <jun.li@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Ruslan Bilovol <ruslan.bilovol@...il.com>,
        Peter Chen <peter.chen@...escale.com>,
        Alan Stern <stern@...land.harvard.edu>,
        grygorii.strashko@...com,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Lee Jones <lee.jones@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Charles Keepax <ckeepax@...nsource.wolfsonmicro.com>,
        patches@...nsource.wolfsonmicro.com,
        Linux PM list <linux-pm@...r.kernel.org>,
        USB <linux-usb@...r.kernel.org>,
        device-mainlining@...ts.linuxfoundation.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v18 0/4] Introduce usb charger framework to deal with the usb gadget power negotation

On Tue, Nov 01 2016, Baolin Wang wrote:


>> So I won't be responding on this topic any further until I see a genuine
>> attempt to understand and resolve the inconsistencies with
>> usb_register_notifier().
>
> Any better solution?

I'm not sure exactly what you are asking, so I'll assume you are asking
the question I want to answer :-)

1/ Liase with the extcon developers to resolve the inconsistencies
  with USB connector types.
  e.g. current there is both "EXTCON_USB" and "EXTCON_CHG_USB_SDP"
  which both seem to suggest a standard downstream port.  There is no
  documentation describing how these relate, and no consistent practice
  to copy.
  I suspect the intention is that
    EXTCON_USB and EXTCON_USB_HOST indicated that data capabilities of
    the cable, while EXTCON_CHG_USB* indicate the power capabilities of
    the cable. 
    So EXTCON_CHG_USB_SDP should always appear together with EXTCON_USB
    while EXTCON_CHG_USB_DCP would not, and EXTCON_CHG_USB_ACA
    would normally appear with EXTCON_USB_HOST (I think).
  Some drivers follow this model, particularly extcon-max14577.c
  but it is not consistent.

  This policy should be well documented and possibly existing drivers
  should be updated to follow it.

  At the same time it would make sense to resolve EXTCON_CHG_USB_SLOW
  and EXTCON_CHG_USB_FAST.  These names don't mean much.
  They were recently removed from drivers/power/axp288_charger.c
  which is good, but are still used in drivers/extcon/extcon-max*
  Possibly they should be changed to names from the standard, or
  possibly they should be renamed to identify the current they are
  expected to provide. e.g. EXTCON_CHG_USB_500MA and EXTCON_CHG_USB_1A

2/ Change all usb phys to register an extcon and to send appropriate
   notifications.  Many already do, but I don't think it is universal.
   It is probable that the extcon should be registered using common code
   instead of each phy driver having its own
   extcon_get_edev_by_phandle()
   or whatever.
   If the usb phy driver needs to look at battery charger registers to
   know what sort of cable was connected (which I believe is the case
   for the chips you are interested in), then it should do that.

3/ Currently some USB controllers discover that a cable was connected by
   listening on an extcon, and some by registering for a usb_notifier
   (described below) ... though there seem to only be 2 left which do that.
   Now that all USB phys send connection information via extcon (see 2),
   the USB controllers should be changed to all find out about the cable
   using extcon.

4/ struct usb_phy contains:
        /* for notification of usb_phy_events */
        struct atomic_notifier_head     notifier;

  This is used inconsistently.  Sometimes the argument passed
  is NULL, sometimes it is a pointer to 'vbus_draw' - the current
  limited negotiated via USB, sometimes it is a pointer the the gadget
  though as far as I can tell, that last one is never used.

  This should be changed to be consistent.  This notifier is no longer
  needed to tell the USB controller that a cable was connected (extcon
  now does that, see 3) so it is only used to communicate the
  'vbus_draw' information.
  So it should be changed to *only* send a notification when vbus_draw
  is known, and it should carry that information.
  This should probably be done in common code, and removed
  from individual drivers.

5/ Now that all cable connection notifications are sent over extcon and
   all vbus_draw notifications are sent over the usb_phy notifier, write
   some support code that a power supply client can use to be told what
   power is available.
   e.g. a battery charger driver would call:
       register_power_client(.....)
   or similar, providing a phandle (or similar) for the usb phy and a
   function to call back when the available current changes (or maybe a
   work_struct containing the function pointer)

   register_power_client() would then register with extcon and separately
   with the usb_phy notifier.  When the different events arrive it
   calculates what ranges of currents are expected and calls the
   call-back function with those details.

6/ Any battery charger that needs to know the available current can now
   call register_power_client() and get the information delivered.

NeilBrown

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