Re: [RFC/PATCH 1/5] usb: otg: add notifier support

[Felipe Balbi]

On Tue, Jan 26, 2010 at 02:35:21PM +0100, ext David Brownell wrote:
> On Tuesday 26 January 2010, Mark Brown wrote:
> > On Tue, Jan 26, 2010 at 03:16:20AM -0800, David Brownell wrote:
> >
> > > I'd vote to convert all the USB-to-charger interfaces so
> > > they use notifiers.  After fixing the events ... see
> > > comments below.  :)
> >
> > Yes please - it's not just chargers either, this can also be used by
> > PMICs which do power path management that includes USB.
>
> Color me confused ... what do you mean by "power path"?
>
> Do you mean something like "the board as a whole can take N mA of
> current from USB", rather than specifically addressing a charger?
>
> It's not uncommon to do things like use VBUS current to power the
> USB circuitry, too.  That can leave less for other purposes.  All
> of that being rather board-specific.
>
>
> > > Those seem like the wrong events.  The right events for a charger
> > > would be more along the lines of:
> >
> > >  - For peripheral:  "you may use N milliAmperes now".
> > >  - General:  "Don't Charge" (a.k.a. "use 0 mA").
> >
> > > I don't see how "N" would be passed with those events ...
> >
> > These are good for the peripheral side.  You do get to pass a void *
> > along with the notifier value, that could be used to pass data like the
> > current limit.
>
> I don't think I saw that being done ... either in code, comments,
> or documentation.  Passing N is fundamental.

yeah, my bad. I should have said that, but it's more related to the 
implementation of the notifier_block.

> > > A host *might* want to be able to say things like "supply
> > > up to N milliAmperes now", e.g. to let a regulator choose
> > > the most efficient mode.
> >
> > Yes, they definitely want this - not just for efficiency but also to
> > allow current limiting to protect the system from excessive current
> > drain.
>
> There are load bursting issues too.  All part of the USB spec;
> a load that's OK for 1 millisecond might not be OK for 1 second.

if you get a SetConfiguration(config), then you can use that load for as 
long as needed, the limitation is when not enumerated, afaict.

> ISTR the "supply N mA" refers to an average.  And there are some
> limits to the capacitance that can practically be stuck on VBUS
> output lines (which includes the cable).  Solvable problems, but
> not always pretty if software has to think it through.
>
> Thing is, supplying current is a bit more involved.  If the
> board can't supply 300 mA, the USB configuration selection
> mechanism has to know that, so it never selects peripheral
> configurations which require that much current.

but that's done already by the usb core, no ? It rules out configuration 
based on the hub->power_budget (can't remember if the field is that 
exact name).

--
balbi
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