RE: [RFC PATCH 2/3] power: supply: Add support for PDOs props

[Adam Thomson <Adam.Thomson.Opensource@diasemi.com>]

On 08 October 2021 12:10, Heikki Krogerus wrote:

> > To downscope this issue for the time being, one of our immediate goals
> > is to expose the PDOs
> > to userspace for metrics reporting and potentially for some power
> > policy control through other
> > channels (like Chrome OS Embedded Controller).
> >
> > Would it be acceptable to revise this series to drop the power supply
> > support for now (since I don't yet
> > see a consensus on how to implement it for the partner), and just add
> > sysfs nodes for each PDO ?
> > This would be akin to how it's being done for identity VDOs right now.
> >
> > So we would have :
> >
> > /sys/class/typec/<port>-partner/source_pdos/pdo{1-13}
> >
> > and
> >
> > /sys/class/typec/<port>-partner/sink_pdos/pdo{1-13}
> >
> > and similarly for the port device.
> >
> > If we want to add additional parsing of the  Fixed Supply PDO into
> > individual properties for the partner/port,
> > those can of course be added later.
> >
> > WDYT?
> 
> I don't think we should use sysfs to expose and control any of these
> objects. It does not really matter under which subsystem we are
> working. Sysfs is just the wrong interface for this kind of data.
> 
> I'm now preparing a proof-of-concept patches where I create character
> device for every USB PD capable device (port, plug and partner). The
> idea is that we could use those char devices to tap into the USB PD
> protocol directly. Right now I'm thinking the nodes would look like
> this (with the first Type-C port):
> 
>         /dev/pd0/port
>         /dev/pd0/plug0 - you only get this node with full featured cables
>         /dev/pd0/plug1 - ditto
>         /dev/pd0/partner - and this is here only if you are connected
> 
> So in this case you would use those char devices to send the actual
> Get_Source_Cap and Get_Sink_Cap messages to get the PDOs.
> 
> The problem is that it's not going to be possible to always support
> every type of command. For example with UCSI we are pretty much
> limited to the capability control messages. But I still think this is
> the right way to do this.
> 
> Let me know what you think.

My two pence worth; this feels like a more appropriate mechanism to access that
data. Look forward to seeing the POC.