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

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Date:   Fri, 19 May 2017 06:27:58 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Badhri Jagan Sridharan <badhri@...gle.com>
Cc:     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 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@...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?
> 
Yes.

Thanks,
Guenter