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

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Message-ID: <2b7e6715-35b8-0d4a-fb28-3a59bd72916e@roeck-us.net>
Date:   Wed, 17 May 2017 02:36:44 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Oliver Neukum <oneukum@...e.com>,
        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 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.

Thanks,
Guenter