linux-kernel - Re: [PATCH v6 5/5] mfd: max77541: Add ADI MAX77541/MAX77540 PMIC Support

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Message-ID: <ZCRG9uilzFjkAtsJ@smile.fi.intel.com>
Date:   Wed, 29 Mar 2023 17:11:02 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Sahin, Okan" <Okan.Sahin@...log.com>
Cc:     Nuno Sá <noname.nuno@...il.com>,
        Mark Brown <broonie@...nel.org>, Lee Jones <lee@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Cosmin Tanislav <demonsingur@...il.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Caleb Connolly <caleb.connolly@...aro.org>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
        "Bolboaca, Ramona" <Ramona.Bolboaca@...log.com>,
        ChiYuan Huang <cy_huang@...htek.com>,
        "Tilki, Ibrahim" <Ibrahim.Tilki@...log.com>,
        William Breathitt Gray <william.gray@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        ChiaEn Wu <chiaen_wu@...htek.com>,
        Haibo Chen <haibo.chen@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>
Subject: Re: [PATCH v6 5/5]  mfd: max77541: Add ADI MAX77541/MAX77540 PMIC
 Support

On Wed, Mar 29, 2023 at 05:08:44PM +0300, Andy Shevchenko wrote:
> On Tue, Mar 28, 2023 at 04:01:21PM +0000, Sahin, Okan wrote:
> > >On Tue, Mar 28, 2023 at 04:51:18PM +0200, Nuno Sá wrote:
> > >> On Tue, 2023-03-28 at 17:35 +0300, Andy Shevchenko wrote:
> > >> > On Tue, Mar 28, 2023 at 04:18:30PM +0200, Nuno Sá wrote:
> > >> > > On Tue, 2023-03-28 at 14:46 +0100, Mark Brown wrote:
> > >> > > > On Tue, Mar 28, 2023 at 03:26:44PM +0200, Nuno Sá wrote:
> > >> > > >
> > >> > > > > IIRC, regmap_read() is not really reentrant and it is used in
> > >> > > > > the IIO driver on the sysfs interface. So, yeah, I think you
> > >> > > > > need the regmap lock and better just leave the config as is.
> > >> > > > > Yes, the lock is
> > >> > > > > opt-
> > >> > > > > out
> > >> > > > > so let's not disable it :)
> > >> > > >
> > >> > > > All the regmap operations are fully thread safe.
> > >> > >
> > >> > > Even if 'config->disable_locking' is set? I think that is what's
> > >> > > being discussed in here...
> > >> >
> > >> > In case the driver has its own lock to serialize IO how on earth the
> > >> > regmap lock is needed. That's what I asked the author of the driver.
> > >> > He told the code
> > >>
> > >> Well, if the driver has it's own locking, then sure we do not need
> > >> regmap's lock...
> > >>
> > >> > doesn't require the regmap lock, and I tend to believe the author.
> > >> > So, why to
> > >> > keep it?
> > >>
> > >> However, if you look at the adc driver, I can see plain regmap_read()
> > >> calls without any "outside" locking.
> > >
> > >Then author of the code should know what they are doing. Right?
> 
> > Actually, I do not want to disable regmap lock that's why I did not update it.
> 
> If you have something like 
> 
> func1()
> 	regmap_read(reg1)
> 	regmap_read/write(reg2)
> 
> func2()
> 	regmap_read/write(regX) // X may or may not be 1 or 2
> 
> and func1() and func2() can be run in parallel then the code is racy.

I have to add that it's racy depending on the hardware of course.
In some cases it may be not a problem, in some it can. _Strictly_
speaking it's racy.

> Do you have such in your code?

Please, double check that. It's recommended to explain your locking schema
somewhere in the code top comment so anybody who reads it later and tries
to modify will know what to expect.

-- 
With Best Regards,
Andy Shevchenko