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

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Message-ID: <20170518165149.GA30223@roeck-us.net>
Date:   Thu, 18 May 2017 09:51:49 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Badhri Jagan Sridharan <badhri@...gle.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, 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 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 ?

Thanks,
Guenter