Date:   Tue, 29 May 2018 08:36:25 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Amelie DELAUNAY <amelie.delaunay@...com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Alexandre TORGUE <alexandre.torgue@...com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/5] dt-bindings: pinctrl: document the STMFX pinctrl
 bindings

On Mon, 28 May 2018, Amelie DELAUNAY wrote:

> On 05/24/2018 09:13 AM, Linus Walleij wrote:
> > On Fri, May 18, 2018 at 9:29 AM, Amelie DELAUNAY <amelie.delaunay@...com> wrote:
> >> On 05/17/2018 08:36 AM, Lee Jones wrote:
> >>> On Wed, 16 May 2018, Amelie DELAUNAY wrote:
> >>>> On 05/16/2018 04:20 PM, Linus Walleij wrote:
> >>>>> On Wed, May 9, 2018 at 9:56 AM, Amelie DELAUNAY <amelie.delaunay@...com> wrote:
> >>>>>
> >>>>>> Indeed, stmfx has other functions than GPIO. But, after comments done
> >>>>>> here: [1] and there: [2], it has been decided to move MFD parent/GPIO
> >>>>>> child drivers into a single PINCTRL/GPIO driver because of the following
> >>>>>> reasons:
> >>>>>> - Other stmfx functions (IDD measurement and TouchScreen controller) are
> >>>>>> not used on any of the boards using an stmfx and supported by Linux, so
> >>>>>> no way to test these functions, and no need to maintain them while they
> >>>>>> are not being used.
> >>>>>> - But, in the case a new board will use more than GPIO function on
> >>>>>> stmfx, the actual implementation allow to easily extract common init
> >>>>>> part of stmfx and put it in an MFD driver.
> >>>>>>
> >>>>>> So I could remove gpio sub-node and put its contents in stmfx node and
> >>>>>> keep single PINCTRL/GPIO driver for the time being.
> >>>>>> Please advise,
> >>>>>
> >>>>> I would normally advice to use the right modeling from the start, create
> >>>>> the MFD driver and spawn the devices from there. It is confusing
> >>>>> if the layout of the driver(s) doesn't really match the layout of the
> >>>>> hardware.
> >>>>>
> >>>>> I understand that it is a pain to write new MFD drivers to get your
> >>>>> things going and it would be "nice to get this working really quick
> >>>>> now" but in my experience it is better to do it right from the start.
> >>>>>
> >>>>
> >>>> Hi Linus,
> >>>>
> >>>> Thanks for your advice. I understand the point.
> >>>> So, the right modeling would be to:
> >>>> - create an MFD driver with the common init part of stmfx
> >>>> - remove all common init part of stmfx-pinctrl driver and keep only all
> >>>> gpio/pinctrl functions.
> >>>>
> >>>> I will not develop the other stmfx functions (IDD measurement driver and
> >>>> TouchScreen controller driver) because, as explained ealier, they are
> >>>> not used on any of the boards using an stmfx and supported by Linux, so
> >>>> no way to test these functions, and no need to maintain them while they
> >>>> are not being used.
> >>>>
> >>>> Lee, are you OK with that ?
> >>>
> >>> I missed a lot of this conversation I think, but from what I've read,
> >>> it sounds fine.
> >>>
> >>
> >> I summarize the situation:
> >> - I still don't have an official datasheet for STMFX device which could
> >> justify the use of an MFD driver;
> >> - the MFD driver will contain the STMFX chip initialization stuff such
> >> as regmap initialization (regmap structure will be shared with the
> >> child), chip initialization, global interrupt management;
> >> - there will be only one child (GPIO/PINCTRL node) for the time being.
> > 
> > But there will be more devices in it. And they will invariably be put
> > to use later, and there will be new versions of the chip as well, and
> > then you will be happy about doing the MFD core, which makes it
> > easy to add new variants with different subdevices.
> > 
> >> So, is "MFD driver + GPIO/PINCTRL driver" the right modeling, and does
> >> it still sound fine after this summary ? :)
> > 
> > No I think it should use an MFD core.
> > 
> > Mainly because of device tree concerns.
> > 
> > The main reason is that the device tree bindings will be different if
> > you add an MFD core later, the GPIO and pinctrl driver will
> > move to a child node, making old device trees incompatible.
> > 
> > We could have a single driver in GPIO+pin control if it is a child
> > of an MFD node in the device tree, but it doesn't make much
> > sense as the I2C device need to be probing to the MFD core.
> > 
> 
> I agree with you Linus, and that's why all STMFX chip initialization 
> stuff was decorrelated in pinctrl-stmfx. This shows that this stuff 
> needs to be in an MFD core.
> 
> But as there is only one child for now (due to the reasons mentioned 
> earlier), it can suggest that it is not a Multi-Function Device.
> 
> I'm not able to target when IDD or TS functions will be required on a 
> Linux product, but it still makes sense to consider that these functions 
> will be used on a Linux product.
> 
> So, I think MFD core + GPIO/pinctrl driver is the right modeling, but I 
> wanted to be sure that this is okay for everyone. I don't want to spend 
> time on something that will not be accepted due to its modeling.

It's fine.  Go ahead.

Thanks for seeing this through to a reasonable conclusion.

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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