Message-ID: <7d7feb374cbf5a587dc1ce65fc3ad672@walle.cc>
Date:   Mon, 08 Jun 2020 17:41:51 +0200
From:   Michael Walle <michael@...le.cc>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Lee Jones <lee.jones@...aro.org>,
        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 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.

>> 
>> > > > 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)

>> > > >  (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.

>> > > 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?

>> 
>> > 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.

Yeah, I agree.

-michael

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