Re: [typec] pd: fixed source capability: peak_current field

[Heikki Krogerus <heikki.krogerus@linux.intel.com>]

+linux-usb ml, Guenter and others.

On Sun, Sep 10, 2023 at 09:29:04PM -0400, Douglas Gilbert wrote:
> Hi,
> The Peak current 2 bit field has been present in the fixed source PDO
> since PD 2.0 . It is commented out in typec/pd.c with a note to revisit
> as it needs the Operational current field from the active RDO.
> 
> Not sure I agree with that: the 2 bit field is unit-less and could easily
> be displayed as such. At the moment the active RDO is not directly
> available through sysfs and would certainly be a useful addition for
> many reasons.
> 
> So my vote (not that I get one) would be to output peak_current in
> sysfs with no unit. Then in the ABI peak_current could be explained
> by reference to a table in the spec (e.g. "USB Power Delivery
> Specification Revision 3.1, Version 1.8, table 6-10 Fixed Power
> Source Peak Current Capability"). Then the issue of what it means
> is handed off to the user space :-)
> 
> Also of note is that the Peak current field is B21..B20 which is
> below B23 (obviously) so all fixed source PDOs have that field
> (and thus those unit-less values could differ), not just
> PDO #1 .

I agree with you. I don't think it makes sense to try to interpret the
peak current value in kernel, so we could just expose the value as-is.

But first giving the others heads-up on this, just in case somebody
disagrees.

thanks,

-- 
heikki