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

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Message-ID: <CAPTae5JS+cNFEkzjT6uQPCiNpc56NUTFAQQovWnQFN9H3RhwUA@mail.gmail.com>
Date:   Thu, 18 May 2017 14:08:53 -0700
From:   Badhri Jagan Sridharan <badhri@...gle.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Oliver Neukum <oneukum@...e.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.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 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.

>
> Thanks,
> Guenter