Date:   Mon, 08 Jun 2020 23:09:01 +0200
From:   Michael Walle <michael@...le.cc>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        david.m.ertman@...el.com, shiraz.saleem@...el.com,
        Rob Herring <robh+dt@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-hwmon@...r.kernel.org, linux-pwm@...r.kernel.org,
        linux-watchdog@...r.kernel.org,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Shawn Guo <shawnguo@...nel.org>, Li Yang <leoyang.li@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v4 02/11] mfd: Add support for Kontron sl28cpld management
 controller

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@...aro.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?

> The issues I wish to resolve using 'simple-mfd' are when sub-devices
> register maps overlap and intertwine.
> 
>> > > > > > 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.
>> 
>> sounds good to me. (although there would then be a probe fail per 
>> sub-device
>> if the version is not supported)
> 
> I don't see an issue with that.  I would put that check inside a
> shared call though, complete with support for locking.
> 
>> > > > > >  (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?
>> 
>> - 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.

-michael

> 
>> > > > > 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'.
>> 
>> see below (1)
>> 
>> > >
>> > > 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.
>> 
>> How does (1) play together with this? What do you propose the "reg"
>> property should contain?
> 
> Whatever is in the 'reg' property contained in the Device Tree node.
> Either the full address or an offset would be suitable.
> 
> Caveat: All this thinking has been done on-the-fly.  I would need to
> look at some examples of existing devices and start coding before I
> could really think the solution through.
> 
> Happy to discuss and/or take recommendations though.

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