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Date:   Tue, 9 Jun 2020 15:42:01 +0100
From:   Mark Brown <broonie@kernel.org>
To:     Michael Walle <michael@walle.cc>
Cc:     Lee Jones <lee.jones@linaro.org>,
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        Linus Walleij <linus.walleij@linaro.org>,
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Subject: Re: [PATCH v4 02/11] mfd: Add support for Kontron sl28cpld
 management controller
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On Tue, Jun 09, 2020 at 04:38:31PM +0200, Michael Walle wrote:

>   mfd-device@10 {
>     compatible =3D "simple-regmap", "simple-mfd";
>     reg =3D <10>;
>     regmap,reg-bits =3D <8>;
>     regmap,val-bits =3D <8>;
>     sub-device@0 {
>       compatible =3D "vendor,sub-device0";
>       reg =3D <0>;
>     };

A DT binding like this is not a good idea, encoding the details of the
register map into the DT binding makes it an ABI which is begging for
trouble.  I'd also suggest that any device using a generic driver like
this should have a specific compatible string for the device so we can
go back and add quirks later if we need them.

>     ...
> };
>=20
> Or if you just want the regmap:
>=20
> &soc {
>   regmap: regmap@fff0000 {
>     compatible =3D "simple-regmap";
>     reg =3D <0xfff0000>;
>     regmap,reg-bits =3D <16>;
>     regmap,val-bits =3D <32>;
>   };
>=20
>   enet-which-needs-syscon-too@1000000 {
>     vendor,ctrl-regmap =3D <&regmap>;
>   };
> };
>=20
> Similar to the current syscon (which is MMIO only..).
>=20
> -michael
>=20
> >=20
> > I can't think of any reasons why not, off the top of my head.
> >=20
> > Does Regmap only deal with shared accesses from multiple devices
> > accessing a single register map, or can it also handle multiple
> > devices communicating over a single I2C channel?
> >=20
> > One for Mark perhaps.
> >=20
> > > > The issues I wish to resolve using 'simple-mfd' are when sub-devices
> > > > register maps overlap and intertwine.
> >=20
> > [...]
> >=20
> > > > > > > What do these bits configure?
> > > > >
> > > > > - hardware strappings which have to be there before the board pow=
ers
> > > > > up,
> > > > >   like clocking mode for different SerDes settings
> > > > > - "keep-in-reset" bits for onboard peripherals if you want to save
> > > > > power
> > > > > - disable watchdog bits (there is a watchdog which is active right
> > > > > from
> > > > >   the start and supervises the bootloader start and switches to
> > > > > failsafe
> > > > >   mode if it wasn't successfully started)
> > > > > - special boot modes, like eMMC, etc.
> > > > >
> > > > > Think of it as a 16bit configuration word.
> > > >
> > > > And you wish for users to be able to view these at run-time?
> > >=20
> > > And esp. change them.
> > >=20
> > > > Can they adapt any of them on-the-fly or will the be RO?
> > >=20
> > > They are R/W but only will only affect the board behavior after a
> > > reset.
> >=20
> > I see.  Makes sense.  This is board controller territory.  Perhaps
> > suitable for inclusion into drivers/soc or drivers/platform.

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