linux-kernel - Re: [PATCH 8/8] gpio: sim: new testing module

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Date:   Mon, 1 Feb 2021 15:30:55 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Joel Becker <jlbec@...lplan.org>,
        Christoph Hellwig <hch@....de>,
        Jonathan Corbet <corbet@....net>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Kent Gibson <warthog618@...il.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-doc <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH 8/8] gpio: sim: new testing module

On Mon, Feb 01, 2021 at 01:53:16PM +0100, Bartosz Golaszewski wrote:
> On Mon, Feb 1, 2021 at 1:49 PM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> > On Mon, Feb 01, 2021 at 11:59:31AM +0100, Bartosz Golaszewski wrote:
> > > On Mon, Feb 1, 2021 at 11:28 AM Andy Shevchenko
> > > <andriy.shevchenko@...ux.intel.com> wrote:
> > > > On Sat, Jan 30, 2021 at 09:37:55PM +0100, Bartosz Golaszewski wrote:
> > > > > On Fri, Jan 29, 2021 at 4:57 PM Andy Shevchenko
> > > > > <andriy.shevchenko@...ux.intel.com> wrote:
> > > > > > On Fri, Jan 29, 2021 at 02:46:24PM +0100, Bartosz Golaszewski wrote:

...

> > > > > > > +struct gpio_sim_chip_config {
> > > > > > > +     struct config_item item;
> > > > > > > +
> > > > > > > +     /*
> > > > > > > +      * If pdev is NULL, the item is 'pending' (waiting for configuration).
> > > > > > > +      * Once the pointer is assigned, the device has been created and the
> > > > > > > +      * item is 'live'.
> > > > > > > +      */
> > > > > > > +     struct platform_device *pdev;
> > > > > >
> > > > > > Are you sure
> > > > > >
> > > > > >         struct device *dev;
> > > > > >
> > > > > > is not sufficient?
> > > > > >
> > > > >
> > > > > It may be but I really prefer those simulated devices to be on the platform bus.
> > > >
> > > > My point here is that there is no need to keep specific bus devices type,
> > > > because you may easily derive it from the struct device pointer. Basically if
> > > > you are almost using struct device in your code (seems to me the case), you
> > > > won't need to carry bus specific one and dereference it each time.
> > >
> > > But don't we need a bus to even register a device? I haven't checked
> > > in a long time but IIRC it's mandatory.
> > >
> > > Let me give you a different argument - the platform device offers a
> > > very simple API for registering devices with properties being
> > > duplicated behind the scenes etc. It seems to me that registering a
> > > bare struct device * would take more boiler-plate code for not much
> > > gain.
> >
> > Yes, I'm not objecting the platform bus choice. I'm objecting the keeping of
> > the pointer to the bus specific structure.
> >
> > There are helpers like to_platform_device() which make the bus specific
> > pointers go away from the structures and easier code when you use exactly
> > pointer to struct device rather than bus specific one.
> >
> 
> Ok I get it. We almost never dereference it though. We do it in probe,
> but there's no way around it. In sysfs callbacks we already get a
> pointer to struct device. And when unregistering the platform device,
> we need to pass it as struct platform_device anyway. I don't see any
> gain from that and would prefer to keep it as is.

It's purely trade off, if you have more *dev in use, better to use *dev, if
*pdev, then use it, although my practice shows that in most cases keeping bus
specific pointer is an overkill.

-- 
With Best Regards,
Andy Shevchenko