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

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Date:   Thu, 25 May 2023 13:31:17 -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: [RFC PATCH v1] usb: core: add sysfs entry for usb device state

On Thu, May 25, 2023 at 12:10 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Thu, May 25, 2023 at 11:46:23AM -0700, Roy Luo wrote:
> > On Thu, May 25, 2023 at 11:02 AM Alan Stern <stern@...land.harvard.edu> wrote:
> > >
> > > On Thu, May 25, 2023 at 05:38:18PM +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.
> > > >
> > > > Signed-off-by: Roy Luo <royluo@...gle.com>
> > > > ---
> > >
> > > > diff --git a/drivers/usb/core/hub.h b/drivers/usb/core/hub.h
> > > > index e23833562e4f..110143568c77 100644
> > > > --- a/drivers/usb/core/hub.h
> > > > +++ b/drivers/usb/core/hub.h
> > > > @@ -84,8 +84,10 @@ struct usb_hub {
> > > >   * @peer: related usb2 and usb3 ports (share the same connector)
> > > >   * @req: default pm qos request for hubs without port power control
> > > >   * @connect_type: port's connect type
> > > > + * @state: device state of the usb device attached to the port
> > >
> > > This member is essentially a duplicate of the .child member of the
> > > usb_port structure.  That is, it points to the .state member of the
> > > child device instead of to the child device itself, but this is pretty
> > > much the same thing.  You could replace *(port_dev->state) with
> > > port_dev->child->state.
> > >
> > Alan, thanks for the quick response!
> > Yes, port_dev->state is indeed the same as port_dev->child->state. However,
> > I still add port_dev->state because port_dev->child won't be assigned until
> > the corresponding usb_device is in ADDRESS state.
> > I wish I can assign get port_dev->child assigned earlier, but I think
> > the current design - assign port_dev->child and device_add() after ADDRESS
> > state - also makes sense because there are many ways that the enumeration
> > could fail in the early stage. By adding port_dev->state, I can link
> > usb_device->state to usb_port as soon as the usb_device is created to get
> > around the limitation of port_dev->child.
> > I would be very happy to hear other ideas.
>
> Is there any real reason not to set port_dev->child as soon as the
> usb_device structure is created?  If enumeration fails, the pointer can
> be cleared.
>
> Alan Stern

Currently the usb core assumes the usb_device that port_dev->child points
to is enumerated and port_dev->child->dev is registered when
port_dev->child is present. Setting port_dev->child early would break this
fundamental assumption, hence I'm a bit reluctant to go this way.