linux-kernel - Re: [PATCH 2/2] i2c: designware: Add support for a bus clock

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Date:   Wed, 18 Jul 2018 13:06:12 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Simon Horman <horms@...ge.net.au>
Cc:     Phil Edworthy <phil.edworthy@...esas.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH 2/2] i2c: designware: Add support for a bus clock

On Wed, Jul 18, 2018 at 11:14 AM Simon Horman <horms@...ge.net.au> wrote:
> On Tue, Jul 17, 2018 at 02:57:27PM +0000, Phil Edworthy wrote:
> > On 17 July 2018 15:47, Andy Shevchenko wrote:
> > > On Tue, 2018-07-17 at 14:40 +0000, Phil Edworthy wrote:
> > > > On 17 July 2018 15:19, Andy Shevchenko wrote:
> > > > > On Tue, 2018-07-17 at 12:42 +0000, Phil Edworthy wrote:
> > > > >
> > > > > > > While your point sounds valid (don't remember how clk_get() is
> > > > > > > implemented), NULL is also OK to have.
> > > > > >
> > > > > > Ok as in there is no bus clock, right?
> > > > > > So it should be:
> > > > > >  if (!IS_ERR_OR_NULL (dev->busclk))
> > > > >
> > > > > Nope, NULL is no error case for optional clock.
> > > >
> > > > I must be missing something here...
> > >
> > > See how clk_prepare_enable() is implemented.
> > Ok, if busclk is NULL the code can safely call clk_prepare_enable()
> >
> > > > I agree that NULL for an optional clock is not an error. However, the
> > > > code above is now:
> > > > + if (prepare) {
> > > > +         /* Optional bus clock */
> > >
> > > > +         if (!IS_ERR_OR_NULL(dev->busclk)) {
> > >
> > > Check for NULL is redundant.
> > >
> > > > +                 ret = clk_prepare_enable(dev->busclk);
> > > > +                 if (ret)
> > > > +                         return ret;
> > > > +         }
> > > > +
> > > >           return clk_prepare_enable(dev->clk);
> > > > + }
> > > >
> > > > So, if you have a valid busclk, it gets enabled, otherwise it is left
> > > > alone.
> > >
> > So the code as sent in the original email is correct (aside from Geert's
> > comments about EPROBE_DEFER handling).
> >
> > Maybe I need some coffee :\
> > Thanks
> > Phil
>
> My point is that errors should be treated as errors.
>
> In i2c_dw_prepare_clk() the following appears:
>
>         if (IS_ERR(dev->clk))
>                 return PTR_ERR(dev->clk);
>
> So dev->clk being an error value is treated as an error that
> is passed up to the caller.
>
> But in your patch (and the snippet below) dev->busclk is treated
> as the optional clock not being present. Even if the error stored
> nothing to do with the clock not being present - f.e.  ENOMEM or
> as Geert mentioned elsewhere, EPROBE_DEFER.
>
> Assuming the absense of the optional clock is indicated by ENOENT,
> in my view correct code would include something like:
>
>         ...
>
>         if (IS_ERR(dev->clk))
>                 return PTR_ERR(dev->clk);
>
>         if (IS_ERR(dev->buslck) && PTR_ERR(dev->busclk) != -ENOENT)
>                 return PTR_ERR(dev->busclk);
>
>         ...

As this is a commonly-used construct, perhaps it would be good to introduce
clk_get_optional() (cfr. gpiod_get_optional()) and devm_clk_get_optional(),
which would return NULL instead of -ENOENT?
Then it becomes a simple check for IS_ERR() in the driver.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds