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

[Michael Walle]

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@xxxxxxxxxx> 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.

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..).

-michael

> 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.
>
> > > The issues I wish to resolve using 'simple-mfd' are when sub-devices
> > > register maps overlap and intertwine.
>
> [...]
>
> > > > > > What do these bits configure?
> > > >
> > > > - hardware strappings which have to be there before the board powers
> > > > 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?
> >
> > And esp. change them.
> >
> > > Can they adapt any of them on-the-fly or will the be RO?
> >
> > They are R/W but only will only affect the board behavior after a 
> > reset.
>
> I see.  Makes sense.  This is board controller territory.  Perhaps
> suitable for inclusion into drivers/soc or drivers/platform.