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Date:   Wed, 10 Jun 2020 19:02:41 +0100
From:   Lee Jones <lee.jones@linaro.org>
To:     Rob Herring <robh+dt@kernel.org>
Cc:     Michael Walle <michael@walle.cc>,
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Subject: Re: [PATCH v4 02/11] mfd: Add support for Kontron sl28cpld
 management controller
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On Wed, 10 Jun 2020, Rob Herring wrote:

> On Wed, Jun 10, 2020 at 1:19 AM Lee Jones <lee.jones@linaro.org> wrote:
> >
> > 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.
> 
> TBC, while fine for a driver to bind on 'simple-mfd', a DT compatible
> with that alone is not fine.

'simple-mfd' essentially means:

  "This device doesn't do anything useful, but the children do."

When used with 'syscon' it means:

  "Memory for this device is shared between all children"

Adding more specific/descriptive compatible strings is conceptually
fine, but they should not be forced to bind to a real driver using
them.  Else we're creating drivers for the sake of creating drivers.

This is especially true with 'simple-mfd' is used without 'syscon'.

> > > 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.
> 
> I think it is really the syscon mfd driver you want to generalize to
> other buses. Though with a quick look at it, there's not really a
> whole lot to share. The regmap lookup would be the main thing. You are
> going to need a driver instance for each bus type.

On it.

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