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

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Message-ID: <87fuujgwsm.fsf@intel.com>
Date:	Mon, 18 Apr 2016 11:12:41 +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:
>> >>> +#define DEFAULT_SDP_CUR_LIMIT        (500 - DEFAULT_CUR_PROTECT)
>> >>
>> >> According to the spec we should always be talking about unit loads (1
>> >> unit load is 100mA for HS/FS/LS and 150mA for SS). Also, this will not
>> >> work for SS capable ports and SS gadgets (we have quite a few of them,
>> >> actually). You're missing the opportunity of charging at 900mA.
>> >
>> > I follow the DCP/SDP/CDP/ACA type's default current limitation and
>> > user can set them what they want.
>> 
>> no, the user CANNOT set it to what they want. If you get enumerated
>> @100mA and the user just decides to set it to 2000mA, s/he could even
>> melt the USB connector. The kernel _must_ prevent such cases.
>
> root should be allowed to do that.

root should not be allowed to put user at risk. The usb connector is a
path to earth ground in most (all?) desktop computers. Allowing root to
put that connector in a situation where it could melt the connector and
short mains should never be allowed.

> 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.

> N900 will simply not charge from .5A.

it used to with original maemo userspace/kernel.

>> 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 ?

-- 
balbi

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