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Date:   Fri, 5 Mar 2021 09:55:41 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Bartosz Golaszewski <brgl@bgdev.pl>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Joel Becker <jlbec@evilplan.org>,
        Christoph Hellwig <hch@lst.de>, Shuah Khan <shuah@kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
        <u.kleine-koenig@pengutronix.de>,
        Kent Gibson <warthog618@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>
Subject: Re: [PATCH v2 08/12] drivers: export device_is_bound()
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On Fri, Mar 05, 2021 at 09:45:41AM +0100, Bartosz Golaszewski wrote:
> On Fri, Mar 5, 2021 at 9:34 AM Greg KH <gregkh@linuxfoundation.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@bgdev.pl> wrote:
> > > >
> > > > From: Bartosz Golaszewski <bgolaszewski@baylibre.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@baylibre.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.

Ick, no, stop there, that's not a "real" device, please do not abuse
platform devices like that, you all know I hate this :(

Use the virtbus code instead perhaps?

> 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.

How will the kernel crash?  What has created the dev_name sysfs file
before it is possible to be read from?  That feels like the root
problem.

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

Please do not create sysfs files before they can be read from :)

thanks,

greg k-h