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

[Adam Thomson]

On 12 December 2018 02:47, Kyle Tso wrote:

> 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.

Yes, I agree it's almost impossible to do anything from software to mitigate
this which is why the HW design has to have protection for this.

> For 2., it is difficult to predict the behaviors from the careless code as well.

Yes, that's also true, but if it's coded with the intention not to select an
option that's potentially higher than the system can handle then we're less
likely to fall foul of over-voltage scenarios in my opinion. By selecting a
PPS APDO with an upper threshold which falls within the board limits, assuming
the code were to accidentally request something higher than the PPS APDO maximum
then the Source should reject this. Just feels a little safer as we're talking
about controlling an external power source. At the end of the day though the
decision lies with the maintainers on this.

> > 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.

Ok, so the system would just stop charging, correct? I guess the calling code
to control the Source/adapter, via TCPM, wouldn't select a value below 4V in that
scenario anyway?

> 
> thanks,
> Kyle