linux-kernel - Re: [PATCH v1] usb: core: add sysfs entry for usb device state

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Date:   Thu, 1 Jun 2023 16:42:16 -0700
From:   Roy Luo <royluo@...gle.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     raychi@...gle.com, badhri@...gle.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Michael Grzeschik <m.grzeschik@...gutronix.de>,
        Bastien Nocera <hadess@...ess.net>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Flavio Suligoi <f.suligoi@...m.it>,
        Douglas Anderson <dianders@...omium.org>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH v1] usb: core: add sysfs entry for usb device state

On Thu, Jun 1, 2023 at 4:21 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Thu, Jun 01, 2023 at 11:04:56PM +0000, Roy Luo wrote:
> > Expose usb device state to userland as the information is useful in
> > detecting non-compliant setups and diagnosing enumeration failures.
> > For example:
> > - End-to-end signal integrity issues: the device would fail port reset
> >   repeatedly and thus be stuck in POWERED state.
> > - Charge-only cables (missing D+/D- lines): the device would never enter
> >   POWERED state as the HC would not see any pullup.
> >
> > What's the status quo?
> > We do have error logs such as "Cannot enable. Maybe the USB cable is bad?"
> > to flag potential setup issues, but there's no good way to expose them to
> > userspace.
> >
> > Why add a sysfs entry in struct usb_port instead of struct usb_device?
> > The struct usb_device is not device_add() to the system until it's in
> > ADDRESS state hence we would miss the first two states. The struct
> > usb_port is a better place to keep the information because its life
> > cycle is longer than the struct usb_device that is attached to the port.
> >
> > Reviewed-by: Alan Stern <stern@...land.harvard.edu>
> > Signed-off-by: Roy Luo <royluo@...gle.com>
> > ---
>
> > diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> > index 97a0f8faea6e..35d94288726b 100644
> > --- a/drivers/usb/core/hub.c
> > +++ b/drivers/usb/core/hub.c
> > @@ -2018,6 +2018,23 @@ bool usb_device_is_owned(struct usb_device *udev)
> >       return !!hub->ports[udev->portnum - 1]->port_owner;
> >  }
> >
> > +static void update_port_device_state(struct usb_device *udev)
> > +{
> > +     struct usb_port *port_dev = NULL;
> > +     struct usb_hub *hub = NULL;
> > +     struct kernfs_node *state_node = NULL;
> > +
> > +     if (udev->parent) {
> > +             hub = usb_hub_to_struct_hub(udev->parent);
> > +             port_dev = hub->ports[udev->portnum - 1];
> > +             WRITE_ONCE(port_dev->state, udev->state);
> > +             state_node = sysfs_get_dirent(port_dev->dev.kobj.sd, "state");
> > +             if (state_node) {
> > +                     sysfs_notify_dirent(state_node);
> > +             }
> > +     }
> > +}
>
> I didn't notice the "= NULL" initializers before.  You might want to
> remove them, since they are completely unnecessary.
>
> Alan Stern

Ack, sending out v2.

Thanks,
Roy