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

From: Guenter Roeck
Date: Fri May 19 2017 - 09:28:08 EST

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@xxxxxxxxxxxx> 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


"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?