Hello David,
On Thu, Apr 12, 2007 at 11:06:46PM -0700, David Brownell wrote:
> > This driver used to stop code/logic duplication through different
> > machines we porting at handhelds.org. pda_power register machs' power
> > supplies, and will take care about notifying batteries about power
> > changes through external power interface.
>
> It gets USB power management wrong though. Have a look at two I2C drivers
> in drivers/i2c/chips: tps65010.c (which would talk to this API) and then
> isp1301_omap.c (which would talk to the tps65010 driver). The tps65010
> chip accepts the same two power sources you're addressing here, as part
> of its battery charging responsibilities. [1]
>
> Key points:
>
> - The API needs to say *how much power* can be drawn. Common values
> are 8 mA (OTG peripherals before configuration), 100 mA (non-OTG ones
> before configuration), 500 mA (high power configurations) ... but the
> exact value depends on what's listed in the configuration descriptor
> for the chosen configuration, any value 0..500 mA (increments of two)
> could be appropriate.
>
> - Sensing VBUS power is not the same thing as being allowed to consume
> it. Again, USB OTG devices are different: OTG hosts **SUPPLY** the
> current, so you really don't want to be telling the OTG transceiver to
> fire up its charge pump (say, 3.0V VBAT converted to 5V VBUS) while at
> the same time telling the battery manager to use that battery-derived
> VBUS current to recharge its battery! Not only would that waste power,
> but it also deprives the peripheral of the power it needs to draw.
>
> In general, no component other than the USB peripheral (or host) controller
> driver has any business trying to control how VBUS power is used. It's
> likely to get it wrong ... as shown by this patch. ;)
>
> And that's exactly why the USB gadget API has had calls to manage the USB
> power consumption since mid-2004. And I wonder why H5000 code evidently
> doesn't implement those calls.
>
> - Dave
>
> [1] You may find http://www.linux-usb.org/gadget/h2-otg.html useful too.
> It gives a high level map of the complicated OTG case, including
> those drivers, and points out how simpler peripheral-only systems
> could behave with the same drivers. However it does turn out to
> be useful if USB peripheral drivers have a "transciever" notion
> that can receive "VBUS present" interrupts, letting the peripheral
> controller driver power down almost *everything* to save power,
> and that otg_transceiver interface does that job.
I see. Current devices I have just consumes power from USB host. I.e.
they able to boot using only USB and discharged battery. :-/ Even
more, HP iPaq hx4700 able to charge battery from USB (using that driver).
We just setting USB charging GPIO, and it starts consume power from
the host.
But I got the point, and yes I can't explain why it works correctly.
(1) Anyway, you're hinting that you'd want to see some
"usb_vbus_start_consume_power(how much)" callback, which will be
handled by gadget driver? Or it should just start using some gadget api?
Or even pda_battery must become an "usb power supplicant" gadget itself
to consume power by law?
-
I'm stuck in null-modem century, so don't wonder my USB-dumbness.
Will read http://www.linux-usb.org/gadget/h2-otg.html, of course.
Though I'd appreciate answers for (1), thus I can think in right
direction from the start.
Much thanks for your comments.
--
Anton Vorontsov
email: [email protected]
backup email: [email protected]
irc://irc.freenode.org/bd2
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