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

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Message-ID: <CAMRc=Md7FeQAd4Syh685+jyZAq2QStBNoo0ACQxrSB=4N6d3dg@mail.gmail.com>
Date:   Fri, 5 Mar 2021 10:16:10 +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 9:55 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> 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@...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.
>
> 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?
>

I have no idea what virtbus is and grepping for it only returns three
hits in: ./drivers/pci/iov.c and it's a function argument.

If it stands for virtual bus then for sure it sounds like the right
thing but I need to find more info on this.

> > 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.
>

It's not sysfs - it's in configfs. Each chip has a read-only configfs
attribute that returns the name of the device - I don't really have a
better idea to map the configfs items to devices that committing
creates.

> > Please advise on how to handle it without device_is_bound().
>
> Please do not create sysfs files before they can be read from :)
>

Don't worry, I don't.

Bartosz