Date:	Mon, 18 Apr 2016 12:23:16 +0200
From:	Pavel Machek <pavel@....cz>
To:	Felipe Balbi <balbi@...nel.org>
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!

> >> >>> +#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.

Be sure that hardware people put reasonable precautions...

> > 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.

> > N900 will simply not charge from .5A.
> 
> it used to with original maemo userspace/kernel.

Yeah, at about 10% per night.

> >> 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.

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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