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Message-ID: <aJsoS3EXgoLP-f-E@kuha.fi.intel.com> Date: Tue, 12 Aug 2025 14:40:59 +0300 From: Heikki Krogerus <heikki.krogerus@...ux.intel.com> To: Abhishek Pandit-Subedi <abhishekpandit@...omium.org> Cc: Andrei Kuchynski <akuchynski@...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 v3 04/10] usb: typec: Expose mode priorities via sysfs Hi Abhishek, On Mon, Aug 11, 2025 at 11:22:38AM -0700, Abhishek Pandit-Subedi wrote: > On Mon, Aug 11, 2025 at 7:25 AM Heikki Krogerus > <heikki.krogerus@...ux.intel.com> wrote: > > > > Hi Andrei, > > > > On Mon, Aug 04, 2025 at 09:03:33AM +0000, Andrei Kuchynski wrote: > > > This patch introduces new sysfs attributes to allow users to configure > > > and view Type-C mode priorities. > > > > > > `priority`, `usb4_priority` attributes allow users to assign a numeric > > > priority to DisplayPort alt-mode, Thunderbolt alt-mode, and USB4 mode. > > > > > > `mode_priorities` - read-only attribute that displays an ordered list > > > of all modes based on their configured priorities. > > > > > > Signed-off-by: Andrei Kuchynski <akuchynski@...omium.org> > > > --- > > > Documentation/ABI/testing/sysfs-class-typec | 33 +++++ > > > drivers/usb/typec/Makefile | 2 +- > > > drivers/usb/typec/class.c | 103 +++++++++++++++- > > > drivers/usb/typec/class.h | 1 + > > > drivers/usb/typec/mode_selection.c | 130 ++++++++++++++++++++ > > > drivers/usb/typec/mode_selection.h | 23 ++++ > > > include/linux/usb/typec_altmode.h | 7 ++ > > > 7 files changed, 295 insertions(+), 4 deletions(-) > > > create mode 100644 drivers/usb/typec/mode_selection.c > > > create mode 100644 drivers/usb/typec/mode_selection.h > > > > > > diff --git a/Documentation/ABI/testing/sysfs-class-typec b/Documentation/ABI/testing/sysfs-class-typec > > > index 38e101c17a00..575dd94f33ab 100644 > > > --- a/Documentation/ABI/testing/sysfs-class-typec > > > +++ b/Documentation/ABI/testing/sysfs-class-typec > > > @@ -162,6 +162,39 @@ Description: Lists the supported USB Modes. The default USB mode that is used > > > - usb3 (USB 3.2) > > > - usb4 (USB4) > > > > > > + What: /sys/class/typec/<port>/<alt-mode>/priority > > > +Date: July 2025 > > > +Contact: Andrei Kuchynski <akuchynski@...omium.org> > > > +Description: > > > + Displays and allows setting the priority for a specific alt-mode. > > > + When read, it shows the current integer priority value. Lower numerical > > > + values indicate higher priority (0 is the highest priority). > > > + If the new value is already in use by another mode, the priority of the > > > + conflicting mode and any subsequent modes will be incremented until they > > > + are all unique. > > > + This attribute is visible only if the kernel supports mode selection. > > > + > > > + What: /sys/class/typec/<port>/usb4_priority > > > +Date: July 2025 > > > +Contact: Andrei Kuchynski <akuchynski@...omium.org> > > > +Description: > > > + Displays and allows setting the priority for USB4 mode. Its behavior and > > > + priority numbering scheme are identical to the general alt-mode > > > + "priority" attributes. > > > > I'm not sure those above two file make any sense. > > > > > +What: /sys/class/typec/<port>/mode_priorities > > > +Date: July 2025 > > > +Contact: Andrei Kuchynski <akuchynski@...omium.org> > > > +Description: This read-only file lists the modes supported by the port, > > > + ordered by their activation priority. It reflects the preferred sequence > > > + the kernel will attempt to activate modes (DisplayPort alt-mode, > > > + Thunderbolt alt-mode, USB4 mode). > > > + This attribute is visible only if the kernel supports mode selection. > > > + > > > + Example values: > > > + - "USB4 Thunderbolt3 DisplayPort" > > > + - "DisplayPort": the port only supports Displayport alt-mode > > > > Why not just use this one instead so that you write the highest > > priority mode to it? > > Feedback from Greg on > https://lore.kernel.org/linux-usb/2025070159-judgingly-baggage-042a@gregkh/: > > "quote": > Multiple value sysfs files are generally frowned apon. sysfs files that > also have to be manually parsed in the kernel are also frowned apon. > Are you _SURE_ there is no other way that you could possibly do this? > > The reason we originally suggested a single "mode priorities" was > because we weren't sure what to do about USB4. Otherwise, it makes > sense to push a priority field to each alt_mode sysfs group and keep > it internally ordered. This is where I really wish we just treated > USB4 as an alternate mode :) I'm probable forgetting something, but I'm pretty sure I already told you guys that I agree, it should be an alt mode. So why not just register a special alt mode for it? Sorry if I missed something. > As such, our current API recommendation looks like the following: > > * On each port, we lay out priorities for all supported alternate modes + USB4. This first part I understand. > * We expose a file to trigger the mode selection task. Reading from it > gives you the current status of mode selection (single value). > * Detailed results from mode entry are pushed to the mode sysfs group > (via entry_results). Converting these to UEVENT is fine but a more > persistent value in debugfs would be useful for debugging. This second part I would really like to handle separately, after we have a solution for the first part. thanks, -- heikki
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