Re: [PATCH] usb: typec: Defer checking of valid power role swap to low level drivers

[Guenter Roeck]

On 05/19/2017 03:35 AM, Heikki Krogerus wrote:
> On Thu, May 18, 2017 at 02:08:53PM -0700, Badhri Jagan Sridharan wrote:
> > On Thu, May 18, 2017 at 9:51 AM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > > On Thu, May 18, 2017 at 11:13:51AM +0200, Oliver Neukum wrote:
> > > > Am Mittwoch, den 17.05.2017, 02:36 -0700 schrieb Guenter Roeck:
> > > > > On 05/17/2017 12:34 AM, Oliver Neukum wrote:
> > > > > > Am Mittwoch, den 17.05.2017, 00:32 -0700 schrieb Badhri Jagan
> > > > > > Sridharan:
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > > "Two independent set of mechanisms are defined to allow a USB Type-C
> > > > > > > DRP to functionally swap power and data roles. When USB PD is
> > > > > > > supported, power and data role swapping is performed as a subsequent
> > > > > > > step following the initial connection process. For non-PD implementations,
> > > > > > > power/data role swapping can optionally be dealt with as part of the initial
> > > > > > > connection process."
> > > > > >
> > > > > > Well, as I read it, without PD once a connection is established, you
> > > > > > are stuck with your role. So it seems to me that blocking a later
> > > > > > attempt to change it makes sense.
> > > > >
> > > > > That seems to be a harsh and not very user friendly reading of the specification.
> > > > >
> > > > > I would argue that the user doesn't care if the partner supports PD or not
> > > > > when selecting a role, and I would prefer to provide an implementation which is
> > > > > as user friendly as possible.
> > > >
> > > > Data role, no question, you are right.
> > > > Power role is a different question. A switch of power role with PD should
> > > > not lead to a disconnect. Any other method might. So equating them does
> > > > not look like a good idea.
> > >
> > > Not really sure I can follow. If a partner does not support PD, there is no
> > > real distinction between data role and power role, or am I missing something ?
> > >
> > > Are you saying that, if a partner does not support PD, user space should
> > > request a data role swap instead, and that this would be acceptable for you ?
> > >
> > > I don't really understand the difference - a data role swap doesn't cause
> > > a disconnect either if the partner supports PD, and it would still result
> > > in a disconnect/reconnect sequence if the partner does not support PD -
> > > but if it works for you, fine with me.
> > >
> > > Badhri, would that work for us ?
> >
> > Yes Geunter that should work as well. Requesting non-pd role swap either through
> > current_power_role or current_data_role is virtually the same.
>
> So if I understood this correctly, we'll skip this change, right?
Yes.

Thanks,
Guenter