linux-kernel - Re: [PATCH] gpio: sim: don't fiddle with GPIOLIB private members

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Message-ID: <CAMRc=Mc0JgPUEpaes7WcbkMu5JyrpLW8N1+bM-+OJaB+pPX4ew@mail.gmail.com>
Date:   Mon, 4 Sep 2023 11:47:54 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Saravana Kannan <saravanak@...gle.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Kent Gibson <warthog618@...il.com>, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH] gpio: sim: don't fiddle with GPIOLIB private members

On Mon, Sep 4, 2023 at 11:40 AM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Mon, Sep 04, 2023 at 11:22:32AM +0200, Bartosz Golaszewski wrote:
> > On Mon, Sep 4, 2023 at 10:59 AM Andy Shevchenko
> > <andriy.shevchenko@...ux.intel.com> wrote:
> > > On Sat, Sep 02, 2023 at 04:40:05PM +0200, Bartosz Golaszewski wrote:
> > > > On Fri, Sep 1, 2023 at 11:10 PM Andy Shevchenko
> > > > <andriy.shevchenko@...ux.intel.com> wrote:
> > > > > On Fri, Sep 01, 2023 at 08:32:40PM +0200, Bartosz Golaszewski wrote:
>
> ...
>
> > > > > > -     /* Used by sysfs and configfs callbacks. */
> > > > > > -     dev_set_drvdata(&gc->gpiodev->dev, chip);
> > > > > > +     /* Used by sysfs callbacks. */
> > > > > > +     dev_set_drvdata(swnode->dev, chip);
> > > > >
> > > > > dev pointer of firmware node is solely for dev links. Is it the case here?
> > > > > Seems to me you luckily abuse it.
> > > >
> > > > I don't think so. If anything we have a helper in the form of
> > > > get_dev_from_fwnode() but it takes reference to the device while we
> > > > don't need it - we know it'll be there because we created it.
> > > >
> > > > This information (struct device of the GPIO device) can also be
> > > > retrieved by iterating over the device children of the top platform
> > > > device and comparing their fwnodes against the one we got passed down
> > > > from probe() but it's just so many extra steps.
> > > >
> > > > Or we can have a getter in gpio/driver.h for that but I don't want to
> > > > expose another interface is we can simply use the fwnode.
> > >
> > > dev pointer in the fwnode strictly speaking is optional. No-one, except
> > > its solely user, should rely on it (its presence and lifetime).
> >
> > Where is this documented? Because just by a quick glance into
> > drivers/base/core.c I can tell that if a device has an fwnode then
> > fwnode->dev gets assigned when the device is created and cleared when
> > it's removed (note: note even attached to driver, just
> > created/removed). Seems like pretty reliable behavior to me.
>
> Yes, and even that member in fwnode is a hack in my opinion. We should not mix
> layers and the idea in the future to get rid of the fwnode_handle to be
> _embedded_ into struct device. It should be separate entity, and device
> instance may use it as a linked list. Currently we have a few problems because
> of the this design mistake.

I don't see how this would work if fwnodes can exist before struct
device is even created. They - after all - represent the actual
physical device hierarchy which may or may not be populated at
run-time depending on many factors.

Once populated, being able to retrieve the software representation of
the device (struct device) from the node from which it was populated
sounds like a reasonable thing to do. What are those problems and are
they even linked to this issue?

>
> The get_dev_from_fwnode() is used only in devlink and I want to keep it that way.
> Nobody else should use it, really.

I don't care all that much, I can get the device from the children of
the platform device. Still comparing fwnodes, though this time the
other way around.

Bartosz

>
> We can discuss with Saravana, but I don't believe he can convince me otherwise.
>
> --
> With Best Regards,
> Andy Shevchenko
>
>