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

Andrei Kuchynski
2025-09-11T13:11:55Z

On Thu, Sep 11, 2025 at 12:41 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Thu, Sep 11, 2025 at 10:46:52AM +0200, Andrei Kuchynski wrote:
> > On Thu, Sep 11, 2025 at 7:07 AM Greg Kroah-Hartman
> > <gregkh@linuxfoundation.org> wrote:
> > >
> > > 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?
> > >
> >
> > The priority can be u32 or u8, but not bool. I use unsigned int as the
> > precise bit count is not important here.
>
> But the range is, that needs to be documented.
>

The priority range will be documented in the ABI

> > Three different priorities are enough for DisplayPort, Thunderbolt,
> > USB4, at least for now. The algorithm is designed to accommodate any
> > number of modes, as it does not rely on predefined MAX_ALTMODE or
> > MAX_PRIORITY values.
>
> Great, then let's use u8 for now.
>

I will change the priority type from unsigned int to u8.

> > > And what happens when it wraps?
> > >
> >
> > If a user sets all priorities (either 2 or 3) to UINT_MAX, the
> > resulting mode sequence will not be as expected due to overflow.
>
> That sounds like a bug :(
>

I will add a check to the function and return -EOVERFLOW if the input
causes the priority values to wrap.

Greg, thank you for the review!

Andrei