linux-kernel - Re: [PATCH v2 4/5] usb: typec: Implement alternate mode priority handling

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Message-ID: <CAMMMRMcOUdo1eSoHv9+M9zR=8WPjKZ9Q0Af3OTQrH78==kqWgQ@mail.gmail.com>
Date: Fri, 5 Sep 2025 13:59:02 +0200
From: Andrei Kuchynski <akuchynski@...omium.org>
To: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc: Abhishek Pandit-Subedi <abhishekpandit@...omium.org>, Benson Leung <bleung@...omium.org>, 
	Jameson Thies <jthies@...gle.com>, Tzung-Bi Shih <tzungbi@...nel.org>, linux-usb@...r.kernel.org, 
	chrome-platform@...ts.linux.dev, Guenter Roeck <groeck@...omium.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>, "Christian A. Ehrhardt" <lk@...e.de>, 
	Venkat Jayaraman <venkat.jayaraman@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/5] usb: typec: Implement alternate mode priority handling

On Fri, Sep 5, 2025 at 12:25 PM Heikki Krogerus
<heikki.krogerus@...ux.intel.com> wrote:
>
> On Mon, Aug 25, 2025 at 02:57:49PM +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@...omium.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;
>
> Couldn't you just always return 0?
>

Priority values for each mode are consistently unique, eliminating the need
for further processing if a duplicate is found.

> > +             }
> > +     }
> > +
> > +     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);
>
> Then this could be:
>
>         device_for_each_child(&port->dev, &alt, increment_duplicated_priority);
>
> right?
>

Unfortunately, the child's list is not in prioritized order. When a
duplicated value is found, the cycle should start for the whole list again.
I definitely need a loop here.

Thanks,
Andrei