Message-ID: <24c2361e0972a29d188cac5f99e9599977d93dca.camel@fi.rohmeurope.com>
Date:   Thu, 14 Jan 2021 10:57:19 +0000
From:   "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>
To:     "lee.jones@...aro.org" <lee.jones@...aro.org>
CC:     "linux@...ck-us.net" <linux@...ck-us.net>,
        "wim@...ux-watchdog.org" <wim@...ux-watchdog.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        linux-power <linux-power@...rohmeurope.com>,
        "geert@...ux-m68k.org" <geert@...ux-m68k.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>
Subject: Re: [PATCH RESEND v6 2/4] mfd: Support ROHM BD9576MUF and BD9573MUF

Hello Lee,

Nice to see you are back :)

On Thu, 2021-01-14 at 10:00 +0000, Lee Jones wrote:
> On Tue, 29 Dec 2020, Vaittinen, Matti wrote:
> 
> > Hello Again peeps,
> > 
> > On Thu, 2020-12-17 at 12:04 +0200, Matti Vaittinen wrote:
> > > On Wed, 2020-12-02 at 15:32 +0200, Matti Vaittinen wrote:
> > > > Hello Lee,
> > > > 
> > > > On Wed, 2020-12-02 at 12:57 +0000, Lee Jones wrote:
> > > > > On Fri, 27 Nov 2020, Vaittinen, Matti wrote:
> > > > > 
> > > > > > Hello Lee,
> > > > > > 
> > > > > > On Fri, 2020-11-27 at 08:32 +0000, Lee Jones wrote:
> > > > > > > On Mon, 23 Nov 2020, Matti Vaittinen wrote:
> > > > > > > 
> > > > > > > > Add core support for ROHM BD9576MUF and BD9573MUF PMICs
> > > > > > > > which
> > > > > > > > are
> > > > > > > > mainly used to power the R-Car series processors.
> > > > > > > > 
> > > > > > > > Signed-off-by: Matti Vaittinen <
> > > > > > > > matti.vaittinen@...rohmeurope.com>
> > > > > > > > ---
> > > > > > > >  drivers/mfd/Kconfig              |  11 ++++
> > > > > > > >  drivers/mfd/Makefile             |   1 +
> > > > > > > >  drivers/mfd/rohm-bd9576.c        | 108
> > > > > > > > +++++++++++++++++++++++++++++++
> > > > > > > >  include/linux/mfd/rohm-bd957x.h  |  59
> > > > > > > > +++++++++++++++++
> > > > > > > >  include/linux/mfd/rohm-generic.h |   2 +
> > > > > > > >  5 files changed, 181 insertions(+)
> > > > > > > >  create mode 100644 drivers/mfd/rohm-bd9576.c
> > > > > > > >  create mode 100644 include/linux/mfd/rohm-bd957x.h
> > > > > > > 
> > > > > > > Looks like a possible candidate for "simple-mfd-i2c".
> > > > > > > 
> > > > > > > Could you look into that please?
> > > > > > > 
> > > > > > I must admit I didn't know about "simple-mfd-i2c". Good
> > > > > > thing
> > > > > > to
> > > > > > know
> > > > > > when working with simple devices :) Is this a new thing?
> > > > > 
> > > > > Yes, it's new.
> > > > > 
> > > > > > I am unsure I understand the idea fully. Should users put
> > > > > > all
> > > > > > the
> > > > > > different regamp configs in this file and just add the
> > > > > > device
> > > > > > IDs
> > > > > > with
> > > > > > pointer to correct config? (BD9576 and BD9573 need volatile
> > > > > > ranges).
> > > > > > Also, does this mean each sub-device should have own node
> > > > > > and
> > > > > > own
> > > > > > compatible in DT to get correctly load and probed? I guess
> > > > > > this
> > > > > > would
> > > > > > need a buy-in from Rob too then.
> > > > > 
> > > > > You should describe the H/W in DT.
> > > > 
> > > > Yes. And it is described. But I've occasionally received
> > > > request
> > > > from
> > > > DT guys to add some properties directly to MFD node and not to
> > > > add
> > > > own
> > > > sub-node. This is what is done for example with the BD71837/47
> > > > clocks
> > > > -
> > > > there is no own node for clk - the clk properties are placed
> > > > directly
> > > > in MFD node (as was requested by Stephen and Rob back then - I
> > > > originally had own node for clk). I really have no clear view
> > > > on
> > > > when
> > > > things warrant for own subnode and when they don't - but as far
> > > > as
> > > > I
> > > > can see using simple-mfd-i2c forces one to always have a sub-
> > > > node /
> > > > device. Even just a empty node with nothing but the compatible
> > > > even
> > > > if
> > > > device does not need stuff from DT? Anyways, I think this is
> > > > nice
> > > > addition for simple drivers.
> > > > 
> > > > > > By the way - for uneducated eyes like mine this does not
> > > > > > look
> > > > > > like
> > > > > > it
> > > > > > has much to do with MFD as a device - here MFD reminds me
> > > > > > of a
> > > > > > simple-
> > > > > > bus on top of I2C.
> > > > > 
> > > > > This is for MFD devices where the parent does little more
> > > > > than
> > > > > create
> > > > > a shared address space for child devices to operate on - like
> > > > > yours.
> > > > > 
> > > > > > Anyways, the BD9576 and BD9573 both have a few interrupts
> > > > > > for
> > > > > > OVD/UVD
> > > > > > conditions and I am expecting that I will be asked to
> > > > > > provide
> > > > > > the
> > > > > > regulator notifiers for those. Reason why I omitted the
> > > > > > IRQs
> > > > > > for
> > > > > > now is
> > > > > > that the HW is designed to keep the IRQ asserted for whole
> > > > > > error
> > > > > > duration so some delayed ack mechanism would be needed. I
> > > > > > would
> > > > > > like to
> > > > > > keep the door open for adding IRQs to MFD core.
> > > > > 
> > > > > You mean to add an IRQ Domain?
> > > > 
> > > > Yes. I planned to use regmap-irq and create irq chip in MFD
> > > > when
> > > > the
> > > > over / under voltage / temperature - notifications or watchdog
> > > > IRQs
> > > > are
> > > > needed. 
> > > 
> > > I am sorry if I have missed your reply. The ROHM email had
> > > redirected
> > > almost all patch emails to spam + I am not sure if some mails are
> > > dropping :(
> > > 
> > > (I am considering moving to gmail - but I'd rather keep all mails
> > > in
> > > one system and I can't transfer work mail traffic to gmail... I
> > > wonder
> > > how others are managing the mails - which mail system you are
> > > using?)
> > > 
> > > I think this series is now pending the decision how to proceed
> > > with
> > > MFD
> > > part. If you still want me to start with "simple-mfd-i2c", then I
> > > would
> > > appreciate if you pointed me how you would like to see the regmap
> > > configs added. Although I am quite positive this (eventually)
> > > ends up
> > > being more than what simple-mfd-i2c is intended for (because at
> > > some
> > > point people want to add the use of the interrupts).
> > 
> > Looking at this topic again. I kind of understand the idea of
> > combining
> > bunch of MFD drivers into one file. Many of the ROHM PMIC MFD
> > drivers
> > do provide same functionality. Regmap configs, regmap IRQ and MFD
> > cells. Some do also probe the device. So having own file for each
> > IC is
> > likely to not scale well when more devices are supported (and I do
> > hope
> > this will be the case also with the ROHM ICs).
> > 
> > What bugs me with the simple-mfd-i2c here is:
> > 1. Requiring to have own compatibles for sub-devices (regulator and
> > WDG) to get them properly probed. (3 compatibles for 1 IC).
> > 2. Requiring to have own DT node for WDG.
> > 3. Supporting differences between BD9576 and BD9573 by having 6
> > compatibles for 2 ICs.
> > 4. Adding interrupt support.
> 
> Linux sees each of these functions as separate devices which are
> handled in different ways by isolated subsystems.  So yes, they each
> require their own compatible string regardless of whether they share
> the same physical piece of silicon or not.

My understanding is that this is exactly why we have MFD? To bring all
the functions under one multifunctional device and to handle things
which are common to all blocks (like IRQs). Besides, like you  know
(better than me) we don't need additional compatibles or dummy dt-nodes 
when MFD instantiates the sub-devices.

> > So ... How do you see adding BD9576/BD9573 MFD stuff in
> > BD9571/(BD9574)
> > MFD driver? The data structures (regmap configs, MFD cells, regmap
> > IRQ
> > portion when added) will be different but the functions and maybe
> > engineers looking at these may be common.
> > 
> > Is it just plain confusing to add core structures for technically
> > different ICs in same file - or is it way to avoid duplicating same
> > code in many files? I can try adding the BD9576/BD9573 to the
> > BD9571
> > core - or I can do resend this as is (rebased on 5.11). I can also
> > hack
> > this to be kicked by simple-mfd-i2c (although I have these strong
> > objections) - but I bet it will in the long run just lead to a sub-
> > optimal solution. When the BD9576/BD9573 logic blocks are re-used
> > in
> > some "non simple" designs and re-using the sub-drivers is needed
> > and/or
> > when IRQs are needed.
> > 
> > (BTW - I am currently working with BD71815/BD71817 - and after this
> > discussion I will add these in BD71828/BD71878 MFD core. I had
> > created
> > new MFD file for them but this discussion has been a nice kick to
> > the
> > better direction for me)
> 
> Everything will be a trade-off.

Yes. The BD71815 is in my opinion something we can and should handle
with same MFD driver as BD71828. 

The BD9576/BD9573 I'd still go with own MFD (as was in this series).
But if you as the maintainer absolutely require me to rework it in some
direction (regardless of my reasoning here) - please let me know. I
would like to get this in a shape that rest of the drivers can also be
accepted in-tree. I think this has been waiting for the decision what
to do with the MFD for a while now.

> There will either be superflouous files or inflexible code.
> 
> You have to make the right decision for the driver and the subsystem.

I think I have explained what I think of but I need you to decide what
is best for the subsystem.


Best regards
	Matti Vaittinen

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