linux-kernel - Re: [PATCH v7 1/4] gadget: Introduce the usb charger framework

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Date:	Mon, 18 Apr 2016 13:30:54 +0300
From:	Felipe Balbi <balbi@...nel.org>
To:	Pavel Machek <pavel@....cz>
Cc:	Baolin Wang <baolin.wang@...aro.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Sebastian Reichel <sre@...nel.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Peter Chen <peter.chen@...escale.com>,
	Alan Stern <stern@...land.harvard.edu>, r.baldyga@...sung.com,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	Lee Jones <lee.jones@...aro.org>,
	Mark Brown <broonie@...nel.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 v7 1/4] gadget: Introduce the usb charger framework


Hi,

Pavel Machek <pavel@....cz> writes:
>> > Very often, you want to charge using 1.8A from an old desktop PC.
>> 
>> if that old desktop's port is not a charging port, you shouldn't be
>> allowed to do that. Not ever.
>
> Yes, Felipe just decided that I should not be able to charge my N900
> in useful way.

you can do whatever you want with *your* kernel binary, but we're not
gonna ship something potentially dangerous. If that PC port is telling
you it can only allow 100mA, you should *not* be allowed to overcome
that limitation from the device side, sorry.

>> >> a) you are connected to a dedicated charger
>> >> 
>> >> 	In this case, you can get up to 2000mA depending on the charger.
>> >> 
>> >> 	If $this charger can give you or not 2000mA is not detectable,
>> >> 	so what do charging ICs do ? They slowly increase the attached
>> >> 	load accross VBUS/GND and measure VBUS value. When IC notices
>> >> 	VBUS dropping bit, step back to previous load.
>> >> 
>> >> 	This means you will always charger with maximum rating of DCP.
>> >> 
>> >> 	Why would user change this ? More is unsafe, less is just
>> >> 	stupid.
>> >
>> > Actually, less is not stupid. Charging li-ion battery from li-ion battery might
>> > be stupid. Imagine I'm on train, with device like N900 (50% battery) and power bank
>> > (3Ah). I'm actively using the device. If I let it charge at full current, I'll waste
>> > energy. If I limit current to approximately the power consumption, it will run the
>> > powerbank empty, first, then empty the internal battery, maximizing total time I
>> > can use the device.
>> 
>> why would you waste energy ? What the charger chip would do is charge
>> battery to maximum then just to maintenance charge from that point
>> on. Where is energy being wasted other than normal heat dissipation ?
>
> Physics 101, of course wasted energy goes to heat. Lets not waste
> energy by charging li-ion from li-ion when it is not required.

your cellphone has no means to know that it's connected to a Li-Ion
battery. We don't have visibility on what we're connected to, just how
much it can source.

-- 
balbi

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