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

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Message-ID: <20170517135120.GG5322@kuha.fi.intel.com>
Date:   Wed, 17 May 2017 16:51:20 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Oliver Neukum <oneukum@...e.com>,
        Badhri Jagan Sridharan <badhri@...gle.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 Wed, May 17, 2017 at 06:02:47AM -0700, Guenter Roeck wrote:
> On 05/17/2017 05:38 AM, Heikki Krogerus wrote:
> > Hi guys,
> > 
> > On Wed, May 17, 2017 at 02:36:44AM -0700, Guenter Roeck wrote:
> > > 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.
> > 
> > I agree. But I have to point out that at least with UCSI, the role
> > swapping is usually not possible without USB PD connection.
> > 
> > So what I'm trying to say is that we can't really promise that role
> > swapping will ever be possible in all cases without PD, which may mean
> > different behavior depending on the platform. I don't know if that is
> > a huge problem.
> > 
> > How predictable do we want this interface to function with role
> > swapping?
> > 
> 
> We can only do as good as we can, but we should not preclude it either.
> PR_SWAP and other swap operations fail a lot in practice with many dongles,
> just because the PD protocol implementation isn't always stable. So even that
> won't be predictable for some time to come.
> 
> As Badhri pointed out earlier, at least some low cost devices won't support PD.
> Since we are talking about high volume products, we _have_ to make it as
> user friendly as we can, if for nothing else to reduce the number of customer
> service calls, repeated bug filings, and, last but not least, to avoid bad press.

OK. So I think we just need to explain in the documentation that there
are no guarantees with role swapping.


Thanks,

-- 
heikki