Re: [PATCH v18 0/4] Introduce usb charger framework to deal with the usb gadget power negotation

[NeilBrown]

On Tue, Dec 20 2016, Baolin Wang wrote:

> Hi Neil,
>
> On 3 November 2016 at 09:25, NeilBrown <neilb@xxxxxxxx> wrote:
>> 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
>
> Now I am creating the new patchset with fixing and converting exist drivers.

Thanks!

>
> I did some investigation about EXTCON subsystem. From your suggestion:
> 1. EXTCON_CHG_USB_SDP should always appear together with EXTCON_USB.
> ---- After checking, now all extcon drivers were following this rule.

what about extcon-axp288.c ?
axp288_handle_chrg_det_event() sets or clears EXTCON_CHG_USB_SDP but
never sets EXTCON_USB.
Similarly phy-rockchip-inno-usb2.c never sets EXTCON_USB.

>
> 2. EXTCON_CHG_USB_ACA would normally appear with EXTCON_USB_HOST.
> ---- Now no extcon drivers used EXTCON_CHG_USB_ACA, then no need to
> change.

Agreed.

>
> 3. Change EXTCON_CHG_USB_SLOW/FAST to EXTCON_CHG_USB_500MA/1A.
> ---- There are no model that shows the slow charger should be 500mA
> and fast charger is 1A. (In extcon-max77693.c file, the fast charger
> can be drawn 2A), so changing to EXTCON_CHG_USB_500MA/1A is not useful
> I think.

Leaving the names a SLOW/FAST is less useful as those names don't *mean*
anything.
The only place where the cable types are registered are in
 extcon-max{14577,77693,77843,8997}.c

In each case, the code strongly suggests that the meaning is that "slow"
means "500mA" and that "fast" means "1A" (or sometimes 1A-2A).

With names like "fast" and "slow", any common changer framework cannot
make use of these cable types as the name doesn't mean anything.
If the names were changed to 500MA/1A, then common code could reasonably
assume how much current can safely be drawn from each.

>
> From above investigation, I did not find anything need to be changed
> now. What do you think? Thanks.
>

As above, I think changes are needed.

I particularly highlight:

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

The documentation is key.  A usb charger framework needs to depend on
correct information from extcon, and currently there is no clear
documentation on how a USB phy should signal the charger type.
There isn't a lot to say: primarily that both the EXTCON_USB* and
EXTCON_CGH_USB* types should be provided together.  Each does not imply
the other in any way.  But it still needs to be documented.

Thanks,
NeilBrown


>>
>> 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
>
>
>
> -- 
> Baolin.wang
> Best Regards

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