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Date:   Tue, 9 Jun 2020 20:45:05 +0100
From:   Lee Jones <lee.jones@linaro.org>
To:     Michael Walle <michael@walle.cc>
Cc:     Andy Shevchenko <andy.shevchenko@gmail.com>,
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        Mark Brown <broonie@kernel.org>,
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        Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Subject: Re: [PATCH v4 02/11] mfd: Add support for Kontron sl28cpld
 management controller
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On Tue, 09 Jun 2020, Michael Walle wrote:

> Am 2020-06-09 17:19, schrieb Lee Jones:
> > On Tue, 09 Jun 2020, Michael Walle wrote:
> > 
> > > Am 2020-06-09 08:47, schrieb Lee Jones:
> > > > On Mon, 08 Jun 2020, Michael Walle wrote:
> > > >
> > > > > Am 2020-06-08 20:56, schrieb Lee Jones:
> > > > > > On Mon, 08 Jun 2020, Michael Walle wrote:
> > > > > >
> > > > > > > Am 2020-06-08 12:02, schrieb Andy Shevchenko:
> > > > > > > > +Cc: some Intel people WRT our internal discussion about similar
> > > > > > > > problem and solutions.
> > > > > > > >
> > > > > > > > On Mon, Jun 8, 2020 at 11:30 AM Lee Jones <lee.jones@linaro.org> wrote:
> > > > > > > > > On Sat, 06 Jun 2020, Michael Walle wrote:
> > > > > > > > > > Am 2020-06-06 13:46, schrieb Mark Brown:
> > > > > > > > > > > On Fri, Jun 05, 2020 at 10:07:36PM +0200, Michael Walle wrote:
> > > > > > > > > > > > Am 2020-06-05 12:50, schrieb Mark Brown:
> > > > > > > >
> > > > > > > > ...
> > > > > > > >
> > > > > > > > > Right.  I'm suggesting a means to extrapolate complex shared and
> > > > > > > > > sometimes intertwined batches of register sets to be consumed by
> > > > > > > > > multiple (sub-)devices spanning different subsystems.
> > > > > > > > >
> > > > > > > > > Actually scrap that.  The most common case I see is a single Regmap
> > > > > > > > > covering all child-devices.
> > > > > > > >
> > > > > > > > Yes, because often we need a synchronization across the entire address
> > > > > > > > space of the (parent) device in question.
> > > > > > > >
> > > > > > > > >  It would be great if there was a way in
> > > > > > > > > which we could make an assumption that the entire register address
> > > > > > > > > space for a 'tagged' (MFD) device is to be shared (via Regmap) between
> > > > > > > > > each of the devices described by its child-nodes.  Probably by picking
> > > > > > > > > up on the 'simple-mfd' compatible string in the first instance.
> > > > > > > > >
> > > > > > > > > Rob, is the above something you would contemplate?
> > > > > > > > >
> > > > > > > > > Michael, do your register addresses overlap i.e. are they intermingled
> > > > > > > > > with one another?  Do multiple child devices need access to the same
> > > > > > > > > registers i.e. are they shared?
> > > > > > >
> > > > > > > No they don't overlap, expect for maybe the version register, which is
> > > > > > > just there once and not per function block.
> > > > > >
> > > > > > Then what's stopping you having each device Regmap their own space?
> > > > >
> > > > > Because its just one I2C device, AFAIK thats not possible, right?
> > > >
> > > > Not sure what (if any) the restrictions are.
> > > 
> > > You can only have one device per I2C address. Therefore, I need one
> > > device
> > > which is enumerated by the I2C bus, which then enumerates its
> > > sub-devices.
> > > I thought this was one of the use cases for MFD. (Regardless of how a
> > > sub-device access its registers). So even in the "simple-regmap"
> > > case this
> > > would need to be an i2c device.
> 
> Here (see below)

Yes, it should still be an I2C device.

> > > 
> > > E.g.
> > > 
> > > &i2cbus {
> > >   mfd-device@10 {
> > >     compatible = "simple-regmap", "simple-mfd";
> > >     reg = <10>;
> > >     regmap,reg-bits = <8>;
> > >     regmap,val-bits = <8>;
> > >     sub-device@0 {
> > >       compatible = "vendor,sub-device0";
> > >       reg = <0>;
> > >     };
> > >     ...
> > > };
> > > 
> > > Or if you just want the regmap:
> > > 
> > > &soc {
> > >   regmap: regmap@fff0000 {
> > >     compatible = "simple-regmap";
> > >     reg = <0xfff0000>;
> > >     regmap,reg-bits = <16>;
> > >     regmap,val-bits = <32>;
> > >   };
> > > 
> > >   enet-which-needs-syscon-too@1000000 {
> > >     vendor,ctrl-regmap = <&regmap>;
> > >   };
> > > };
> > > 
> > > Similar to the current syscon (which is MMIO only..).
> > 
> > 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().

> > > > I can't think of any reasons why not, off the top of my head.
> > > >
> > > > 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?
> > > >
> > > > One for Mark perhaps.

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