Re: [PATCH v3 4/5] usb: typec: Implement alternate mode priority handling

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Sep 10, 2025 at 07:35:42PM +0000, Andrei Kuchynski wrote:
> On Wed, Sep 10, 2025 at 1:31 PM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Fri, Sep 05, 2025 at 02:22:05PM +0000, Andrei Kuchynski wrote:
> > > This patch introduces APIs to manage the priority of USB Type-C alternate
> > > modes. These APIs allow for setting and retrieving a priority number for
> > > each mode. If a new priority value conflicts with an existing mode's
> > > priority, the priorities of the conflicting mode and all subsequent modes
> > > are automatically incremented to ensure uniqueness.
> > >
> > > Signed-off-by: Andrei Kuchynski <akuchynski@chromium.org>
> > > ---
> > >  drivers/usb/typec/Makefile         |  2 +-
> > >  drivers/usb/typec/mode_selection.c | 38 ++++++++++++++++++++++++++++++
> > >  drivers/usb/typec/mode_selection.h |  6 +++++
> > >  include/linux/usb/typec_altmode.h  |  1 +
> > >  4 files changed, 46 insertions(+), 1 deletion(-)
> > >  create mode 100644 drivers/usb/typec/mode_selection.c
> > >  create mode 100644 drivers/usb/typec/mode_selection.h
> > >
> > > diff --git a/drivers/usb/typec/Makefile b/drivers/usb/typec/Makefile
> > > index 7a368fea61bc..8a6a1c663eb6 100644
> > > --- a/drivers/usb/typec/Makefile
> > > +++ b/drivers/usb/typec/Makefile
> > > @@ -1,6 +1,6 @@
> > >  # SPDX-License-Identifier: GPL-2.0
> > >  obj-$(CONFIG_TYPEC)          += typec.o
> > > -typec-y                              := class.o mux.o bus.o pd.o retimer.o
> > > +typec-y                              := class.o mux.o bus.o pd.o retimer.o mode_selection.o
> > >  typec-$(CONFIG_ACPI)         += port-mapper.o
> > >  obj-$(CONFIG_TYPEC)          += altmodes/
> > >  obj-$(CONFIG_TYPEC_TCPM)     += tcpm/
> > > diff --git a/drivers/usb/typec/mode_selection.c b/drivers/usb/typec/mode_selection.c
> > > new file mode 100644
> > > index 000000000000..2179bf25f5d4
> > > --- /dev/null
> > > +++ b/drivers/usb/typec/mode_selection.c
> > > @@ -0,0 +1,38 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +/*
> > > + * Copyright 2025 Google LLC.
> > > + */
> > > +
> > > +#include "mode_selection.h"
> > > +#include "class.h"
> > > +#include "bus.h"
> > > +
> > > +static int increment_duplicated_priority(struct device *dev, void *data)
> > > +{
> > > +     struct typec_altmode **alt_target = (struct typec_altmode **)data;
> > > +
> > > +     if (is_typec_altmode(dev)) {
> > > +             struct typec_altmode *alt = to_typec_altmode(dev);
> > > +
> > > +             if (alt != *alt_target && alt->priority == (*alt_target)->priority) {
> > > +                     alt->priority++;
> > > +                     *alt_target = alt;
> > > +                     return 1;
> > > +             }
> > > +     }
> > > +
> > > +     return 0;
> > > +}
> > > +
> > > +void typec_mode_set_priority(struct typec_altmode *alt,
> > > +             const unsigned int priority)
> > > +{
> > > +     struct typec_port *port = to_typec_port(alt->dev.parent);
> > > +     int res = 1;
> > > +
> > > +     alt->priority = priority;
> > > +
> > > +     while (res)
> > > +             res = device_for_each_child(&port->dev, &alt,
> > > +                             increment_duplicated_priority);
> > > +}
> > > diff --git a/drivers/usb/typec/mode_selection.h b/drivers/usb/typec/mode_selection.h
> > > new file mode 100644
> > > index 000000000000..cbf5a37e6404
> > > --- /dev/null
> > > +++ b/drivers/usb/typec/mode_selection.h
> > > @@ -0,0 +1,6 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +
> > > +#include <linux/usb/typec_altmode.h>
> > > +
> > > +void typec_mode_set_priority(struct typec_altmode *alt,
> > > +             const unsigned int priority);
> > > diff --git a/include/linux/usb/typec_altmode.h b/include/linux/usb/typec_altmode.h
> > > index b3c0866ea70f..571c6e00b54f 100644
> > > --- a/include/linux/usb/typec_altmode.h
> > > +++ b/include/linux/usb/typec_altmode.h
> > > @@ -28,6 +28,7 @@ struct typec_altmode {
> > >       int                             mode;
> > >       u32                             vdo;
> > >       unsigned int                    active:1;
> > > +     unsigned int                    priority;
> >
> > What is the range of this?  And this value is only incremented, never
> > decremented?
> >
> 
> The range extends from 0 to UINT_MAX. The value is only incremented.
> The only exception is that If the user sets UINT_MAX for two alternate
> modes in turn, the priority of the first mode becomes 0. This does not
> break the algorithm, and the user can check all priorities via
> ‘priority’ attributes.

Why not use u32 to define a sane range?  Or u8?  How many different
priorities will actually be used in the real world?

And what happens when it wraps?

> I am unsure if a check for this specific case is necessary, as it
> would require examining priorities across all modes, not just a simple
> 'if' statement.
> There are a few ideas in this algorithm:
> - all priorities must always be valid and unique

Is that true?  Where is that validated?

> - no unnecessary restrictions for the user

What does the user actually want to do here?

> - as simple as possible

But not too simple :)

thanks,

greg k-h