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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
