Re: [PATCH] usb: typec: tcpm: Extend the matching rules on PPS APDO selection

From: Kyle Tso
Date: Tue Dec 11 2018 - 21:47:05 EST

On Mon, Dec 10, 2018 at 7:36 PM Adam Thomson
<Adam.Thomson.Opensource@xxxxxxxxxxx> wrote:
> On 10 December 2018 09:01, Adam Thomson wrote:
> > On 06 December 2018 03:02, Kyle Tso wrote:
> >
> > > Current matching rules ensure that the voltage range of selected
> > > Source Capability is entirely within the range defined in one of the
> > > Sink Capabilities. This is reasonable but not practical because Sink
> > > may not support wide range of voltage when sinking power while Source
> > > could advertise its capabilities in raletively wider range. For
> > > example, a Source PDO advertising 3.3V-11V@3A (9V Prog of Fixed
> > > Nominal Voltage) will not be selected if the Sink requires 5V- 12V@3A
> > > PPS power. However, the Sink could work well if the requested voltage range in
> > RDOs is 5V-11V@3A.
> >
> > Is there a real world example of a sink requiring the 5V - 12V range? In that
> > scenario could we not add an additional sink capability which allows for this range
> > to be supported, and the current implementation should work just fine?
> Ok, I maybe should have waited until after my morning coffee to respond. So
> because the lower limit on the sink side, is higher than the advertised source's
> PPS minimum voltage it never gets selected? Personally I'd prefer to keep the
> upper limit checking as is as I think that's an additional safety benefit
> helping to prevent over-voltage scenarios. I think if a PPS APDO can supply up
> to 11V then the system should be capable of handling that voltage, otherwise
> it shouldn't be considered at all. The Source provides limits checking as well
> to make sure the Sink doesn't request a value above the maximum voltage limit
> for that selected APDO.

If the over-voltage occurs, it means:
1. the adapter malfunctioned. or
2. the code on the Sink accidentally requests a voltage level which is
over the limit of the Sink.

For 1., it is difficult to predict the behaviors of a malfunctioned
adapter. The over-voltage event may happen even if the Sink doesn't
select the APDO from this broken adapter.
For 2., it is difficult to predict the behaviors from the careless code as well.

> For the lower limit I'm more inclined to agree with allowing a higher minimum
> on the sink side as that's less of a safety/damage issue as I understand it.
> FWIW, what is the real world scenario? What happens if voltage drops below 5V?

Some products (in Sink mode) have under-voltage protection (the lower
bound might be around 3.8V - 4V before
the calculation of IR-drop) that will cause the disconnection.