linux-kernel - Re: [PATCH v1] iio: magnetometer: ak8974: Silence deferred-probe error

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Message-ID: <CACRpkdbMF4=-g2ic_SKgOkd6kfgKJqZ2UxCRaoXJjq0EiEn+pw@mail.gmail.com>
Date:   Thu, 16 Apr 2020 18:51:54 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio <linux-iio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH v1] iio: magnetometer: ak8974: Silence deferred-probe error

On Thu, Apr 16, 2020 at 4:45 PM Dmitry Osipenko <digetx@...il.com> wrote:
> 16.04.2020 14:33, Linus Walleij пишет:

> > This misses some important aspects of dev_dbg(), notably this:
> >
> > #if defined(CONFIG_DYNAMIC_DEBUG)
> > #define dev_dbg(dev, fmt, ...)                                          \
> >         dynamic_dev_dbg(dev, dev_fmt(fmt), ##__VA_ARGS__)
> > #elif defined(DEBUG)
> > #define dev_dbg(dev, fmt, ...)                                          \
> >         dev_printk(KERN_DEBUG, dev, dev_fmt(fmt), ##__VA_ARGS__)
> > #else
> > #define dev_dbg(dev, fmt, ...)                                          \
> > ({                                                                      \
> >         if (0)                                                          \
> >                 dev_printk(KERN_DEBUG, dev, dev_fmt(fmt), ##__VA_ARGS__); \
> > })
> > #endif
> >
> > If DEBUG is not defined the entire dev_dbg() message is enclodes in if (0)
> > and compiled out of the kernel, saving space. The above does not
> > fulfil that.
>
> Hello Linus,
>
> After some recent discussions in regards to the EPROBE_DEFER handling,
> Thierry Reding suggested the form which is used in my patch and we
> started to use it recently in the Tegra DRM driver [1]. The reason is
> that we don't want to miss any deferred-probe messages under any
> circumstances, for example like in a case of a disabled DYNAMIC_DEBUG.

I have a hard time to accept this reasoning.

Who doesn't feel that way about their subsystem? If you don't want
to miss the message under any circumstances then use dev_info().
Don't override the default behaviour of dev_dbg().

> The debug messages are usually disabled in a release-build and when not
> a very experienced person hands you KMSG for diagnosing a problem, the
> KMSG is pretty much useless if error is hidden silently.

So use dev_info().

> By moving the message to a debug level, we reduce the noise in the KMSG
> because usually people look for a bold-red error messages. Secondly, we
> don't introduce an additional overhead to the kernel size since the same
> text is reused for all error conditions.

dev_info() is not supposed to be an error message, it is supposed to
be information, so use that.

Yours,
Linus Walleij