linux-kernel - Re: [PATCH] usb: typec: mux: Fix matching with typec_altmode

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Date:   Mon, 17 May 2021 18:37:50 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jun Li <jun.li@....com>, Hans de Goede <hdegoede@...hat.com>,
        USB <linux-usb@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: typec: mux: Fix matching with typec_altmode_desc

On Mon, May 17, 2021 at 6:14 PM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
> On Mon 17 May 04:13 CDT 2021, Andy Shevchenko wrote:
> > On Sun, May 16, 2021 at 6:47 AM Bjorn Andersson
> > <bjorn.andersson@...aro.org> wrote:
> > >
> > > In typec_mux_match() "nval" is assigned the number of elements in the
> > > "svid" fwnode property, then the variable is used to store the success
> > > of the read and finally attempts to loop between 0 and "success" - i.e.
> > > not at all - and the code returns indicating that no match was found.
> > >
> > > Fix this by using a separate variable to track the success of the read,
> > > to allow the loop to get a change to find a match.

...

> > > -       nval = fwnode_property_read_u16_array(fwnode, "svid", val, nval);
> > > -       if (nval < 0) {
> > > +       ret = fwnode_property_read_u16_array(fwnode, "svid", val, nval);
> > > +       if (ret < 0) {
> > >                 kfree(val);
> > > -               return ERR_PTR(nval);
> > > +               return ERR_PTR(ret);
> > >         }
> >
> > This changes the behaviour of the original code, i.e. nval can be
> > still positive but less than we got from previous call. Some fwnode
> > backends in some cases potentially can _successfully_ read less than
> > asked.
> >
> > Perhaps
> >
> >   nval = ret;
> >
> > or drop the patch.
> >
>
> Per the kerneldoc of fwnode_property_read_u16_array:
>
>  * Return: number of values if @val was %NULL,
>  *         %0 if the property was found (success),
>
> @val is not NULL, as we just checked for that, so the function will
> always return 0 on success.
>
> I don't see anything indicating that the number of elements can be
> different from what fwnode_property_count_u16() returned.

Okay, I have checked the backends of fwnode and indeed, OF case (from
where I remember such behaviour) deliberately does

if (ret >= 0)
  return 0;

Otherwise the rest return 0 directly / explicitly.

The only exception is _read_string_array().

> > >         for (i = 0; i < nval; i++) {

-- 
With Best Regards,
Andy Shevchenko