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

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Message-ID: <20170519103551.GD14626@kuha.fi.intel.com>
Date:   Fri, 19 May 2017 13:35:51 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Badhri Jagan Sridharan <badhri@...gle.com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Oliver Neukum <oneukum@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        USB <linux-usb@...r.kernel.org>
Subject: Re: [PATCH] usb: typec: Defer checking of valid power role swap to
 low level drivers

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@...ck-us.net> 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?


Thanks,

-- 
heikki