linux-kernel - Re: [PATCH] usb: typec: Add bus type for plug alt modes

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Message-ID: <20201209171524.GK680328@kuha.fi.intel.com>
Date:   Wed, 9 Dec 2020 19:15:24 +0200
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Prashant Malani <pmalani@...omium.org>
Cc:     "open list:USB NETWORKING DRIVERS" <linux-usb@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Benson Leung <bleung@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: typec: Add bus type for plug alt modes

Hi Prashant,

On Wed, Dec 09, 2020 at 08:22:52AM -0800, Prashant Malani wrote:
> Hi Heikki,
> 
> On Wed, Dec 9, 2020 at 8:14 AM Heikki Krogerus
> <heikki.krogerus@...ux.intel.com> wrote:
> >
> > On Tue, Dec 08, 2020 at 03:45:19PM -0800, Prashant Malani wrote:
> > > Hi Heikki,
> > >
> > > Thanks a lot for looking at the patch.
> > >
> > > On Tue, Dec 8, 2020 at 1:37 AM Heikki Krogerus <heikki.krogerus@...ux.intel.com> wrote:
> > > >
> > > > On Wed, Dec 02, 2020 at 07:08:47PM -0800, Prashant Malani wrote:
> > > > > Add the Type C bus for plug alternate modes which are being
> > > > > registered via the Type C connector class. This ensures that udev events
> > > > > get generated when plug alternate modes are registered (and not just for
> > > > > partner/port alternate modes), even though the Type C bus doesn't link
> > > > > plug alternate mode devices to alternate mode drivers.
> > > >
> > > > I still don't understand how is the uevent related to the bus? If you
> > > > check the device_add() function, on line 2917, kobject_uevent() is
> > > > called unconditionally. The device does not need a bus for that event
> > > > to be generated.
> > >
> > > My initial thought process was to see what is the difference in the adev device
> > > initialization between partner altmode and plug altmode (the only difference I saw in
> > > typec_register_altmode() was regarding the bus field).
> > >
> > > Yes, kobject_uevent() is called unconditionally, but it's return value isn't checked,
> > > so we don't know if it succeeded or not.
> > >
> > > In the case of cable plug altmode, I see it fail with the following error[1]:
> > >
> > > [  114.431409] kobject: 'port1-plug0.0' (000000004ad42956): kobject_uevent_env: filter function caused the event to drop!
> > >
> > > I think the filter function which is called is this one: drivers/base/core.c: dev_uevent_filter() [2]
> > >
> > > static int dev_uevent_filter(struct kset *kset, struct kobject *kobj)
> > > {
> > >       struct kobj_type *ktype = get_ktype(kobj);
> > >
> > >       if (ktype == &device_ktype) {
> > >               struct device *dev = kobj_to_dev(kobj);
> > >               if (dev->bus)
> > >                       return 1;
> > >               if (dev->class)
> > >                       return 1;
> > >       }
> > >       return 0;
> > > }
> > >
> > > So, both the "if (dev->bus)" and "if (dev->class)" checks are failing here. In the case of partner alt modes, bus is set by the class.c code
> > > so this check likely returns 1 in that case.
> >
> > OK. I understand the issue now. So I would say that the proper
> > solution to this problem is to link the alt modes with the class
> > instead of the bus. That is much smaller change IMO.
> 
> Got it. Just to confirm that I understand correctly, do you mean:
> 1. Only cable plug alt modes should be linked with the class instead of the bus.
> 
> <or>
> 
> 2. All alt modes (cable plug, partner, port) should be linked with the
> class instead of the bus
> 
> My initial interpretation is 1.) since the bus linkage would be
> necessary to match alt mode drivers to partner alt mode devices.
> But, my understanding of the bus code is limited so I could be wrong;
> could you kindly clarify?

We don't need to care about the bus here. A device can be part of a
bus and a class at the same time. I don't think there is any reason to
limit the class to only plug alt modes, so let's just assign it to all
of them.

thanks,

-- 
heikki