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Subject: Re: [PATCH v1] iio: magnetometer: ak8974: Silence deferred-probe
 error
To:     Linus Walleij <linus.walleij@linaro.org>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        Hartmut Knaack <knaack.h@gmx.de>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
        linux-iio <linux-iio@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thierry Reding <thierry.reding@gmail.com>
References: <20200414222713.32660-1-digetx@gmail.com>
 <CACRpkdY_J8e127etFFYkoxLDDkc334Xgg8ZbapdU36oGsaZ08g@mail.gmail.com>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <e08c487c-5c2a-3172-7c9c-0e7d2cd51769@gmail.com>
Date:   Thu, 16 Apr 2020 17:45:46 +0300
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16.04.2020 14:33, Linus Walleij пишет:
> Hi Dmitry,
> 
> thanks for your patch!
> 
> On Wed, Apr 15, 2020 at 12:27 AM Dmitry Osipenko <digetx@gmail.com> wrote:
> 
>> It's not uncommon that voltage regulator becomes available later during
>> kernel's boot process, in this case there is no need to print a noisy
>> error message. This patch moves the message about unavailable regulator
>> to the debug level in a case of the deferred-probe error and also amends
>> the message with error code.
>>
>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>> ---
>>  drivers/iio/magnetometer/ak8974.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/iio/magnetometer/ak8974.c b/drivers/iio/magnetometer/ak8974.c
>> index d32996702110..cc3861f97d42 100644
>> --- a/drivers/iio/magnetometer/ak8974.c
>> +++ b/drivers/iio/magnetometer/ak8974.c
>> @@ -718,6 +718,7 @@ static const struct regmap_config ak8974_regmap_config = {
>>  static int ak8974_probe(struct i2c_client *i2c,
>>                         const struct i2c_device_id *id)
>>  {
>> +       const char *level = KERN_ERR;
>>         struct iio_dev *indio_dev;
>>         struct ak8974 *ak8974;
>>         unsigned long irq_trig;
>> @@ -746,7 +747,11 @@ static int ak8974_probe(struct i2c_client *i2c,
>>                                       ARRAY_SIZE(ak8974->regs),
>>                                       ak8974->regs);
>>         if (ret < 0) {
>> -               dev_err(&i2c->dev, "cannot get regulators\n");
>> +               if (ret == -EPROBE_DEFER)
>> +                       level = KERN_DEBUG;
>> +
>> +               dev_printk(level, &i2c->dev, "cannot get regulators: %d\n",
> 
> 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.
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.

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.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e32c8c2a5fbe1514e4ae9063a49e76ecf486ffb8

I can change this patch to something like this:

if (ret == -EPROBE_DEFER)
	dev_dbg("deferring probe\n");
else
	dev_err("cannot...");

but in general this should be a less useful variant, don't you agree?