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Date:   Wed, 20 Feb 2019 23:28:36 +0300
From:   Yauhen Kharuzhy <jekhor@gmail.com>
To:     Hans de Goede <hdegoede@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        MyungJoo Ham <myungjoo.ham@samsung.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Subject: Re: [PATCH 2/2] extcon intel-cht-wc: Enable external charger
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On Wed, Feb 20, 2019 at 05:42:28PM +0100, Hans de Goede wrote:
> > > The input-current-limit only specifies how much current the charger may
> > > draw from the micro-usb for both supplying the laptop as well as for
> > > charging the battery combined. You can safely set this as high as
> > > the charger can handle (2A for a dedicated charger).
> > > 
> > > The BQ25892 should have a *separate* setting for the max current to
> > > use while charging the battery (assuming that the input current allows
> > > drawing enough current in the first place). I would hope that those bits
> > > have some sane value set from the firmware...
> > 
> > Yes, the charger has separate battery current limit but firmware doesn't
> > change its default value (2048 mA) while Lenovo's software driver does.
> > It set battery charging limit to 4 A and input limit to 2 A (it makes
> > sense because Lenovo adapter and BQ25892 both support voltage increasing
> > upto 12V).
> 
> Hmm, I guess your device uses a separate power-barrel charging conector
> then? 12v over micro-usb requires special negotiation which the Whiskey Cove
> PMIC does not support AFAIK.

The Yoga Book supports kind of quick charging by negotiate voltage with
'current pulse protocol' supported by BQ25892 and Lenovo's wall cube.
I think that PMIC is not connected to VBUS directly. After charging
started, BQ25892 driver can ask power adapter to increase voltage
number of times upto 12V. See
https://github.com/jekhor/yogabook-linux-kernel/blob/master/drivers/power/supply/bq25890_charger.c#L918
for example.

> 
> In either case if you want to increase the max battery current to 4A
> in the kernel, then this will have to be guarded by a DMI check.

Yes.

> 
> I beieve the way to do this wuld be throuh a device-property on the
> charger which gets set from drivers/i2c/busses/i2c-cht-wc.c, but as
> said this needs to be behind a DMI check, e cannot just g and boost
> the max charge current to 4A everywhere.

Yes, I use such method already in the my kernel:
https://github.com/jekhor/yogabook-linux-kernel/blob/master/drivers/i2c/busses/i2c-cht-wc.c#L247

Anyway, charger tweaking will be next iteration, now I want to complete
current extcon stuff/

> Regards,
> 
> Hans

-- 
Yauhen Kharuzhy