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

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Date:	Tue, 22 Mar 2016 12:30:12 +0100
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.

Very often, you want to charge using 1.8A from an old desktop PC.

N900 will simply not charge from .5A.

> 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.

Best regards,
										Pavel