linux-kernel - Re: [PATCH v5 1/7] pwm: pca9685: Switch to atomic API

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Date:   Sat, 27 Mar 2021 16:54:47 +0100
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Sven Van Asbroeck <thesven73@...il.com>, linux-pwm@...r.kernel.org,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Lee Jones <lee.jones@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        David Jander <david@...tonic.nl>
Subject: Re: [PATCH v5 1/7] pwm: pca9685: Switch to atomic API

Hi Thierry,

On Mon, Mar 22, 2021 at 08:58:35AM +0100, Thierry Reding wrote:
> On Thu, Dec 17, 2020 at 12:10:10PM -0500, Sven Van Asbroeck wrote:
> > On Thu, Dec 17, 2020 at 11:48 AM Clemens Gruber
> > <clemens.gruber@...ruber.com> wrote:
> > >
> > > I can initialize the values to 0 of course and check the file for other
> > > places with missing initializations.
> > >
> > > Or would it be better to check the return codes of regmap_read/write in
> > > such cases? I'm not sure.
> > 
> > I think that checking the regmap_read/write return values is overkill
> > in this driver. These functions can't realistically fail, except if the i2c
> > bus is bad, i.e. h/w failure or intermittency. And that's an externality
> > which I believe we can ignore.
> 
> I think there are (rare) occasions where it's fine to not check for
> errors, i.e. if you definitively know that calls can't fail. However,
> given that this uses regmap and you don't really know what's backing
> this, I think it's always better to err on the side of caution and
> properly check the return values.
> 
> The fact that this can be externally caused is actually a reason why
> we shouldn't be ignoring any errors. If there's a chip that's hogging
> the I2C bus or if you've even just mistyped the I2C client's address
> in DT, it's better if the PWM driver tells you with an error message
> than if it is silently ignoring the errors and keeps you guessing at
> why the PWM isn't behaving the way it should.
> 
> Granted, the error code isn't always going to pinpoint exactly what's
> going wrong, but for serious errors often the I2C bus driver will let
> you know with an extra error message. However, it's much easier to go
> looking for that error message if the PWM driver lets you know that
> something went wrong.
> 
> Please just add full checking of regmap operations.

OK, I will create a separate patch adding these checks in the next
series.

This will lead to > 20 additional dev_err statements, let me know if I
should instead just return the error code and not add dev_err's for
every failed regmap operation.

Clemens