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

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Message-ID: <ZPWr3dRP5C1GSY9F@smile.fi.intel.com>
Date:   Mon, 4 Sep 2023 13:05:17 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
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 04, 2023 at 11:47:54AM +0200, Bartosz Golaszewski wrote:
> 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.

That's whole idea behind swnodes. They (ideally) should be created _before_
any other object they are being used with. This is how it works today.

And doing swnode->dev = ... contradicts a lot: layering, lifetime objects, etc.

> They - after all - represent the actual
> physical device hierarchy which may or may not be populated at
> run-time depending on many factors.

No. This is a mistaken assumption.

> 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.

Fine, but do not use dev pointer from fwnode, esp. software node.

> > We can discuss with Saravana, but I don't believe he can convince me otherwise.

-- 
With Best Regards,
Andy Shevchenko