linux-kernel - Re: [PATCH v4 1/3] mfd: Add new mfd device TPS68470

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Message-ID: <CAHp75VffOr0Bs9jXoyF_FACfZCgD+D+tyE7tE_cdVUgZxqQn1g@mail.gmail.com>
Date:   Fri, 28 Jul 2017 12:16:11 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     "Mani, Rajmohan" <rajmohan.mani@...el.com>
Cc:     Lee Jones <lee.jones@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alexandre Courbot <gnurou@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        "sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>
Subject: Re: [PATCH v4 1/3] mfd: Add new mfd device TPS68470

On Fri, Jul 28, 2017 at 3:30 AM, Mani, Rajmohan <rajmohan.mani@...el.com> wrote:
>> On Wed, Jul 26, 2017 at 11:23 AM, Lee Jones <lee.jones@...aro.org> wrote:
>> > On Tue, 25 Jul 2017, Andy Shevchenko wrote:
>> >> On Tue, Jul 25, 2017 at 12:10 PM, Lee Jones <lee.jones@...aro.org> wrote:
>>
>> >> I briefly checked few ->read() and ->write() implementations and
>> >> didn't find any evidence of positive numbers that can be returned.
>> >> Documentation (kernel doc) doesn't shed a light on that. So, to me it
>> >> sounds unspecified.
>> >>
>> >> So, for now (until documentation will be fixed) I would rely on if
>> >> (ret < 0)
>> >
>> > It's not unspecified.  The regmap methods call into regcache_write(),
>> > where the kerneldoc is clear.
>>
>
> Since, we are interested in the regmap for the I2C bus here, I looked into the implementation of
>  __devm_regmap_init()
>         __regmap_init()
>                 regcache_init()
> for I2C bus.
>
> At the end of __devm_regmap_init() call from devm_regmap_init_i2c() inside tps68470_probe(), I see that both cache_bypass and defer_caching flags of i2c regmap struct are set. So, it looks regcache_write/read calls do not come into play here.
>
> So, regmap_write()
>         _regmap_write()
>                 map->reg_write (drivers/base/regmap/regmap.c:1665) translates to
>                 regmap_i2c_write(drivers/base/regmap/regmap-i2c.c:128)
>
> These checks in regmap_i2c_write() ensure all return values from i2c_master_send() other than the requested number of bytes to write, are converted into negative values.
>
>         if (ret == count)
>                 return 0;
>         else if (ret < 0)
>                 return ret;
>         else
>                 return -EIO;
>
> Similar argument goes for regmap_read() as well.
> With that, for regmap over I2C bus, it sounds like 'if (ret < 0)' looks to be a better choice. Please see if I missed anything here.

It prooves exactly the Lee's point.

So, perhaps the best approach is to move to
if (ret)
 return ret;

...if it will be a problem in the future, fix it accordingly.

-- 
With Best Regards,
Andy Shevchenko