linux-kernel - Re: [PATCH v4 02/11] mfd: Add support for Kontron sl28cpld management controller

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Message-ID: <20200610183053.GV4106@dell>
Date:   Wed, 10 Jun 2020 19:30:53 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Michael Walle <michael@...le.cc>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        david.m.ertman@...el.com, shiraz.saleem@...el.com,
        Rob Herring <robh+dt@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-hwmon@...r.kernel.org, linux-pwm@...r.kernel.org,
        linux-watchdog@...r.kernel.org,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Shawn Guo <shawnguo@...nel.org>, Li Yang <leoyang.li@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v4 02/11] mfd: Add support for Kontron sl28cpld
 management controller

On Wed, 10 Jun 2020, Michael Walle wrote:

> Am 2020-06-10 09:56, schrieb Lee Jones:
> > On Wed, 10 Jun 2020, Michael Walle wrote:
> > 
> > > Am 2020-06-10 09:19, schrieb Lee Jones:
> > > > On Wed, 10 Jun 2020, Michael Walle wrote:
> > > > > Am 2020-06-09 21:45, schrieb Lee Jones:
> > > > > > On Tue, 09 Jun 2020, Michael Walle wrote:
> > > > > > > > We do not need a 'simple-regmap' solution for your use-case.
> > > > > > > >
> > > > > > > > Since your device's registers are segregated, just split up the
> > > > > > > > register map and allocate each sub-device with it's own slice.
> > > > > > >
> > > > > > > I don't get it, could you make a device tree example for my
> > > > > > > use-case? (see also above)
> > > > > >
> > > > > >     &i2cbus {
> > > > > >         mfd-device@10 {
> > > > > >             compatible = "simple-mfd";
> > > > > >             reg = <10>;
> > > > > >
> > > > > >             sub-device@10 {
> > > > > >                 compatible = "vendor,sub-device";
> > > > > >                 reg = <10>;
> > > > > >             };
> > > > > >    };
> > > > > >
> > > > > > The Regmap config would be present in each of the child devices.
> > > > > >
> > > > > > Each child device would call devm_regmap_init_i2c() in .probe().
> > > > >
> > > > > Ah, I see. If I'm not wrong, this still means to create an i2c
> > > > > device driver with the name "simple-mfd".
> > > >
> > > > Yes, it does.
> > > >
> > > > > Besides that, I don't like this, because:
> > > > >  - Rob already expressed its concerns with "simple-mfd" and so on.
> > > >
> > > > Where did this take place?  I'd like to read up on this.
> > > 
> > > In this thread:
> > > https://lore.kernel.org/linux-devicetree/20200604211039.12689-1-michael@walle.cc/T/#m16fdba5962069e7cd4aa817582ee358c9fe2ecbf
> > > 
> > > >
> > > > >  - you need to duplicate the config in each sub device
> > > >
> > > > You can have a share a single config.
> > > >
> > > > >  - which also means you are restricting the sub devices to be
> > > > >    i2c only (unless you implement and duplicate other regmap configs,
> > > > >    too). For this driver, SPI and MMIO may be viable options.
> > > >
> > > > You could also have a shared implementation to choose between different
> > > > busses.
> > > 
> > > Then what is the difference between to have this shared config in the
> > > parent driver only and use the functions which are already there, i.e.
> > > dev_get_regmap(parent). But see, below, I'll wait with what you're
> > > coming up.
> > 
> > The difference is the omission of an otherwise pointless/superfluous
> > driver.  Actually, it's the difference between the omission of 10
> > pointless drivers!
> 
> If you want to omit anything generic in the device tree - and as far as
> I understand it - that should be the way to go, the specific compatible
> string of the parent device has to go somewhere. Thus I'd appreciate
> a consolidated (MFD) driver which holds all these, as you say it
> pointless drivers.
> Because IMHO they are not pointless, rather they are
> the actual drivers for the MFD. Its sub nodes are just an implementation
> detail to be able to use the OF bindings
> (like your clock example or
> a phandle to a PWM controller). Just because it is almost nothing there
> except the regmap instantiation doesn't mean it is not a valid MFD driver.

A valid MFD driver is whatever we (the Linux community at large)
define it to be.  An MFD is not a real thing.  We made it up.  It's
MFD which is the implementation detail, not the child devices.  If a
driver a) does very little, and b) the very little it does do can be
resolved in a different way, is not a valid driver.  It's a waste of
disk space.

> And there is also additional stuff, like clock enable, version checks, etc.

As more functionality is added *then* we can justify a driver.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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