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SubjectRe: [PATCH v4 02/11] mfd: Add support for Kontron sl28cpld management controller
+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?
>
> > > > But, there is more in my driver:
> > > > (1) there is a version check
>
> If we can rid the Regmap dependency, then creating an entire driver to
> conduct a version check is unjustifiable. This could become an inline
> function which is called by each of the sub-devices instead, for
> example.
>
> > > > (2) there is another function for which there is no suitable linux
> > > > subsystem I'm aware of and thus which I'd like to us sysfs
> > > > attributes for: This controller supports 16 non-volatile
> > > > configuration bits. (this is still TBD)
>
> There is a place for everything in Linux.
>
> What do these bits configure?
>
> > > TBH I'd also say that the enumeration of the subdevices for this
> > > device should be in the device rather than the DT, they don't
> > > seem to be things that exist outside of this one device.
> >
> > We're going circles here, formerly they were enumerated in the MFD.
> > Yes, they are devices which aren't likely be used outside a
> > "sl28cpld", but there might there might be other versions of the
> > sl28cpld with other components on different base addresses. I
> > don't care if they are enumerated in DT or MFD, actually, I'd
> > prefer the latter. _But_ I would like to have the device tree
> > properties for its subdevices, e.g. the ones for the watchdog or
> > whatever components there might be in the future.
>
> [...]
>
> > MFD core can
> > match a device tree node today; but only one per unique compatible
> > string. So what should I use to differentiate the different
> > subdevices?
>
> Right. I have been aware of this issue. The only suitable solution
> to this would be to match on 'reg'.
>
> FYI: I plan to fix this.
>
> If your register map needs to change, then I suggest that this is
> either a new device or at least a different version of the device and
> would also have to be represented as different (sub-)mfd_cell.
>
> > Rob suggested the internal offset, which I did here.
>
> FWIW, I don't like this idea. DTs should not have to be modified
> (either in the first instance or subsequently) or specifically
> designed to patch inadequacies in any given OS.
>
> > But then, there is less use in duplicating the offsets in the MFD
> > just to have the MFD enumerate the subdevices and then match
> > the device tree nodes against it. I can just use
> > of_platform_populate() to enumerate the children and I won't
> > have to duplicate the base addresses.
>
> Which is fine. However this causes a different issue for you. By
> stripping out the MFD code you render the MFD portion seemingly
> superfluous. Another issue driver authors commonly contend with.

--
With Best Regards,
Andy Shevchenko

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