Re: [PATCH v16 0/4] Introduce usb charger framework to deal with the usb gadget power negotation

From: Felipe Balbi <balbi@kernel.org>
To: NeilBrown <nfbrown@novell.com>,
	Baolin Wang <baolin.wang@linaro.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	Sebastian Reichel <sre@kernel.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>,
	David Woodhouse <dwmw2@infradead.org>
Cc: robh@kernel.org, Jun Li <jun.li@nxp.com>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Ruslan Bilovol <ruslan.bilovol@gmail.com>,
	Peter Chen <peter.chen@freescale.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	r.baldyga@samsung.com, grygorii.strashko@ti.com,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>,
	Lee Jones <lee.jones@linaro.org>, Mark Brown <broonie@kernel.org>,
	Charles Keepax <ckeepax@opensource.wolfsonmicro.com>,
	patches@opensource.wolfsonmicro.com,
	Baolin Wang <baolin.wang@linaro.org>,
	Linux PM list <linux-pm@vger.kernel.org>,
	USB <linux-usb@vger.kernel.org>,
	device-mainlining@lists.linuxfoundation.org,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v16 0/4] Introduce usb charger framework to deal with the usb gadget power negotation
Date: Tue, 06 Sep 2016 10:40:48 +0300	[thread overview]
Message-ID: <87poohmq67.fsf@linux.intel.com> (raw)
In-Reply-To: <8760q9a8m6.fsf@notabene.neil.brown.name>

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Hi,

NeilBrown <nfbrown@novell.com> writes:
> Firstly, you have made the current limit associated with each cable type
> configurable (__usb_charger_set_cur_limit_by_type).   This is nonsense.
> The standard (e.g. BC-1.2) declares what the current limits are.  There
> is no reason for those not to be hard coded.

I had raised the same concern WRT configuration current limits.

> Secondly, you treat each charger type as having a single "cur_limit" and
> utilize that limit by telling the PMIC to draw that much current.
> Again, this is inconsistent with the specification.
> BC-1.2 defines, for each charger type, a minimum and maximum current
> level.
> The minimum should always be available.   Attempting to draw more than
> that (but less that the max) might work or might cause the charger
> to shut down, but you can be sure that the charger will respond to the
> increased load by first reducing the voltage, and will not shut down
> until the voltage has dropped below 2V.
> If you try to draw more current than the maximum, then the charger might
> shut down before the voltage drops below 2V.

Very well put :-)

> Given this understanding of the current available from the charger,
> there are two approaches the PMIC might take.
> 1/ if the PMIC is able to exercise fine control over the current it
>   draws, and if it can monitor the voltage on the charger, then it
>   could gradually increase the power being requested until the voltage
>   drops below some threshold (e.g. 4.75V), and then (probably) back off
>   a little.  It should only increase at most up to the maximum, even if
>   the voltage remains high.  It should probably start at zero rather
>   than at the minimum.  This allows for lossage elsewhere.

That's what most charging control SW I've seen in the past ends up
doing. Correct

> 2/ If the PMIC cannot measure voltage, or doesn't have sufficiently fine
>    control of the current requested, then it should request only the
>    minimum available current.  Any more is not safe.

correct

-- 
balbi

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