force usb charge current

[Andy Green]

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Somebody in the thread at some point said:
| On Tue, Jan 27, 2009 at 18:47, Andy Green <andy at openmoko.com> wrote:
|> There is a global "at the USB connector" current limit, but it only has
|> those steps, 100, 500, or 1000mA.
|>
|> You could add ability to force max charging current separately, and
|> estimate a maximum current for "everything else" but it doesn't seem
|> like a good idea to me when WLAN can add >100mA, GSM unit might decide
|> to do something, etc and you only had a budget of 350mA to start with.
|>
|
| Ok.
|
| Is this charging current limit setting already available with current
kernel ?

We just force it to 1A limit in the bootloader, since we rely on the
master USB current limit to limit total current coming in, and the
constant current phase of the charging to determine how much current to
actually use... the PMU automatically splits what we have between
charging and powering the regulators (VB_SYS) that provide power for the
device.

| Anyway it looks better than nothing. For example setting max battery
| charge current to 100mA should allow to not continuously draw too much
| through usb, and allow to keep battery charge.

Hum I don't really love the idea of exposing it, since it feels like the
point we are being too tricksy for our own good: but it doesn't increase
danger on USB side due to the USB total current limit thing you can
already force, and since we max it out by default it doesn't increase
danger on battery side.  The only bad thing you can do with it is
basically defeat the charging action by reducing the max charging
current down to nothing, but even that action is reset in the bootloader
next boot.

Balaji, what do you think?  It basically means allowing setting of MBCC5
with a sys name like force_charging_limit_dangerous.

| And when battery is near empty, not limiting charge current probably
| means usb current will be max (ie 500mA) continuously....
|
| This would not avoid possibility to exceed 350mA, but at least not
always...
|
| When the phone calls for more current (GSM,...), will it first claim
| from usb if max (500) is not yet reached, then from battery, or the
| opposite.... or it just depends on... different things :-p ?

The GSM RF amp is hooked direct to the battery, so if it was charging at
that time it actually wanted to make a call, would drain the battery and
eat whatever charging current was available.

- -Andy
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