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

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Message-ID: <20200610071940.GS4106@dell>
Date:   Wed, 10 Jun 2020 08:19:40 +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 2020-06-09 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.

>  - 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.

> Thus, I'd rather implement a simple-mfd.c which implement a common
> I2C driver for now and populate its children using
> devm_of_platform_populate(). This could be extended to support other
> type of regmaps like SPI in the future.
> 
> Also some MFD drivers could be moved to this, a likely candidate is
> the smsc-ece1099.c. Although I don't really understand its purpose,
> if don't have CONFIG_OF.
> 
> Judging from the existing code, this simple-mfd.c wouldn't just be
> "a list of compatible" strings but also additional quirks and tweaks
> for particular devices in this list.

Hold off on the simple-mfd.c idea, as I'm not taken by it yet and
wouldn't want you to waste your time.  I have another idea which would
help.  Give me a few days to put something together.

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