linux-kernel - Re: [PATCH v2 08/12] drivers: export device_is

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Message-ID: <CAMRc=MddDb+nakgEM+Xeqm=rMMkkWO2EDekD36EoPJashYP88w@mail.gmail.com>
Date:   Fri, 5 Mar 2021 09:45:41 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Joel Becker <jlbec@...lplan.org>,
        Christoph Hellwig <hch@....de>, Shuah Khan <shuah@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Kent Gibson <warthog618@...il.com>,
        Jonathan Corbet <corbet@....net>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH v2 08/12] drivers: export device_is_bound()

On Fri, Mar 5, 2021 at 9:34 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Fri, Mar 05, 2021 at 09:18:30AM +0100, Geert Uytterhoeven wrote:
> > CC Greg
> >
> > On Thu, Mar 4, 2021 at 11:30 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> > >
> > > From: Bartosz Golaszewski <bgolaszewski@...libre.com>
> > >
> > > Export the symbol for device_is_bound() so that we can use it in gpio-sim
> > > to check if the simulated GPIO chip is bound before fetching its driver
> > > data from configfs callbacks in order to retrieve the name of the GPIO
> > > chip device.
> > >
> > > Signed-off-by: Bartosz Golaszewski <bgolaszewski@...libre.com>
> > > ---
> > >  drivers/base/dd.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > > index 9179825ff646..c62c02e3490a 100644
> > > --- a/drivers/base/dd.c
> > > +++ b/drivers/base/dd.c
> > > @@ -353,6 +353,7 @@ bool device_is_bound(struct device *dev)
> > >  {
> > >         return dev->p && klist_node_attached(&dev->p->knode_driver);
> > >  }
> > > +EXPORT_SYMBOL_GPL(device_is_bound);
>
> No.  Please no.  Why is this needed?  Feels like someone is doing
> something really wrong...
>
> NACK.
>

I should have Cc'ed you the entire series, my bad.

This is the patch that uses this change - it's a new, improved testing
module for GPIO using configfs & sysfs as you (I think) suggested a
while ago:

https://lkml.org/lkml/2021/3/4/355

The story goes like this: committing the configfs item registers a
platform device. As far as I understand - there's no guarantee that
the device will be bound to a driver before the commit callback (or
more specifically platform_device_register_full() in this case)
returns so the user may try to retrieve the name of the device
immediately (normally user-space should wait for the associated uevent
but nobody can force that) by doing:

mv /sys/kernel/config/gpio-sim/pending/foo /sys/kernel/config/gpio-sim/live/
cat /sys/kernel/config/gpio-sim/live/foo/dev_name

If the device is not bound at this point, we'll have a crash in the
kernel as opposed to just returning -ENODEV.

Please advise on how to handle it without device_is_bound().

Best Regards,
Bartosz