On Tue, Nov 17, 2020 at 09:40:16AM -0800, Prashant Malani wrote: > Hi Heikki, > > On Tue, Nov 17, 2020 at 02:41:43PM +0200, Heikki Krogerus wrote: > > On Mon, Nov 16, 2020 at 12:11:42PM -0800, Prashant Malani wrote: > > > Add a field to the typec_plug struct to record the number of available > > > altmodes as well as the corresponding sysfs attribute to expose this to > > > userspace. > > > > > > This allows userspace to determine whether there are any > > > remaining alternate modes left to be registered by the kernel driver. It > > > can begin executing any policy state machine after all available > > > alternate modes have been registered with the connector class framework. > > > > > > This value is set to "-1" initially, signifying that a valid number of > > > alternate modes haven't been set for the plug. The sysfs file remains > > > hidden as long as the attribute value is -1. > > > > Why couldn't we just keep it hidden for as long as the number of > > alt modes is 0? If you already explained that, then I apologise, I've > > forgotten. > > > > No worries :) > > Succinctly, because 0 is a valid value for "number of altmodes > supported". > > If we keep the attribute hidden for 0, then there won't > be a way for userspace to determine that PD discovery is done and we > don't expect any more cable plug altmodes to be registered by the kernel > Type C port driver (it can determine this by comparing > "number_of_altmodes" against the actual number of alt modes registered > by the Type C port driver). > > If we keep "number_of_altmodes" hidden even for 0, the userspace cannot > differentiate between "this cable doesn't support any altmodes" and > "it does altmodes, but the PD stack hasn't completed PD Discovery > including DiscoverIdentity yet". > > For reference, here is the initial patch and mini-discussion around it > back in July for port-partner altmodes [1] (I've followed a similar > logic here). > > Hope this helps the rationale a bit more. Got it. Thanks for the explanation (again :-). Reviewed-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx> > Best regards, > > -Prashant > > [1]: > https://lore.kernel.org/linux-usb/20200701082230.GF856968@xxxxxxxxxxxxxxxxx/ thanks, -- heikki
