Re: [PATCH v16 0/4] Introduce usb charger framework to deal with the usb gadget power negotation

From: NeilBrown
Date: Fri Sep 09 2016 - 17:20:02 EST

On Fri, Sep 09 2016, Baolin Wang wrote:

>> In practice, the USB PHY and the Power manager will often be in the same
>> IC (the PMIC) so the driver for one could look at the registers for the
>> other.
>> But there is no guarantee that the hardware works like that. It is
>> best to create a generally design.
> Yes, we hope to create one generally design, so we need to consider
> this situation: the power supply getting the charger type by accessing
> PMIC registers. The registers which save the charger type are not
> always belong to the USB PHY, may be just some registers on PMIC.

If the power_supply can directly detect the type of charger, then it
doesn't need any usb-charger-infrastructure to tell it. It can handle
current selection entirely internally.
Surely the only interesting case for a framework to address is the one
where the power_supply cannot directly detect the charger type.

> Now in mainline kernel, there are 3 methods can get the charger type
> which need to integrate with USB charger framework:
> 1. power supply
> 2. extcon (need to add as you suggested)
> 3. others (by 'get_charger_type' callback of USB charger)

There is no "get_charger_type" now in the mainline kernel.

>>> Suppose the USB configuration requests 100mA, then we should set the
>>> USB charger current is 100mA by __usb_charger_set_cur_limit_by_type()
>>> funtion, then notify this to power driver.
>> ahh.... I had missed something there. It's a while since I looked
>> closely at these patches.
>> Only.... this usage of usb_charger_set_cur_limit_by_type() is really
>> nonsensical.
>> If the cable is detected as being DCP or CDP or ACA (or ACA/DOCK) then
>> the number negotiated with the USB configuration is not relevant and
>> should be ignored. There is a guaranteed minimum which is at least the
>> maximum that *can* be negotiated.
> Yes. If it is not relevant, we will no't set the current from USB
> configuration. Just when your charger type is SDP and the USB
> enumeration is done, we can get the USB configuration from host to set
> current.

But you do!
The mA number from the USB configuration is passed to usb_gadget_vbus_draw.
Your patch passes that to usb_charger_set_cur_limit_by_type()
which calls __usb_charger_set_cur_limit_by_type() which will set the
cur_limit for whichever type uchger->type currently is.

So when it is not relevant, your code *does* set some current limit.


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