linux-kernel - Re: [PATCH v2 3/4] dt-bindings: mfd: Add Delta TN48M CPLD drivers bindings

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Message-ID: <CA+HBbNFW4NHpcEt7Xa+qC2HVvhuKu1O+PUo_rSE03qtSsBCehg@mail.gmail.com>
Date:   Tue, 1 Jun 2021 11:06:05 +0200
From:   Robert Marko <robert.marko@...tura.hr>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     Rob Herring <robh@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Luka Perkov <luka.perkov@...tura.hr>, jmp@...phyte.org,
        Paul Menzel <pmenzel@...gen.mpg.de>,
        Donald Buczek <buczek@...gen.mpg.de>
Subject: Re: [PATCH v2 3/4] dt-bindings: mfd: Add Delta TN48M CPLD drivers bindings

On Tue, Jun 1, 2021 at 10:19 AM Lee Jones <lee.jones@...aro.org> wrote:
>
> On Mon, 31 May 2021, Robert Marko wrote:
>
> > On Wed, May 26, 2021 at 9:52 AM Lee Jones <lee.jones@...aro.org> wrote:
> > >
> > > On Tue, 25 May 2021, Robert Marko wrote:
> > >
> > > > On Tue, May 25, 2021 at 9:46 AM Lee Jones <lee.jones@...aro.org> wrote:
> > > > >
> > > > > On Mon, 24 May 2021, Rob Herring wrote:
> > > > >
> > > > > > On Mon, May 24, 2021 at 02:05:38PM +0200, Robert Marko wrote:
> > > > > > > Add binding documents for the Delta TN48M CPLD drivers.
> > > > > > >
> > > > > > > Signed-off-by: Robert Marko <robert.marko@...tura.hr>
> > > > > > > ---
> > > > > > > Changes in v2:
> > > > > > > * Implement MFD as a simple I2C MFD
> > > > > > > * Add GPIO bindings as separate
> > > > > >
> > > > > > I don't understand why this changed. This doesn't look like an MFD to
> > > > > > me. Make your binding complete if there are missing functions.
> > > > > > Otherwise, stick with what I already ok'ed.
> > > > >
> > > > > Right.  What else, besides GPIO, does this do?
> > > >
> > > > It currently does not do anything else as hwmon driver was essentially
> > > > NACK-ed for not exposing standard attributes.
> > >
> > > Once this provides more than GPIO capabilities i.e. becomes a proper
> > > Multi-Function Device, then it can use the MFD framework.  Until then,
> > > it's a GPIO device I'm afraid.
> > >
> > > Are you going to re-author the HWMON driver to conform?
> > hwmon cannot be reathored as it has no standard hwmon attributes.
> >
> > >
> > > > The CPLD itself has PSU status-related information, bootstrap related
> > > > information,
> > > > various resets for the CPU-s, OOB ethernet PHY, information on the exact board
> > > > model it's running etc.
> > > >
> > > > PSU and model-related info stuff is gonna be exposed via a misc driver
> > > > in debugfs as
> > > > we have user-space SW depending on that.
> > > > I thought we agreed on that as v1 MFD driver was exposing those directly and
> > > > not doing anything else.
> > >
> > > Yes, we agreed that creating an MFD driver just to expose chip
> > > attributes was not an acceptable solution.
> > >
> > > > So I moved to use the simple I2C MFD driver, this is all modeled on the sl28cpld
> > > > which currently uses the same driver and then GPIO regmap as I do.
> > > >
> > > > Other stuff like the resets is probably gonna get exposed later when
> > > > it's required
> > > > to control it directly.
> > >
> > > In order for this driver to tick the MFD box, it's going to need more
> > > than one function.
> >
> > Understood, would a debug driver count or I can expose the resets via
> > a reset driver
> > as we have a future use for them?
>
> CPLDs and FPGAs are funny ones and are often difficult to support in
> Linux.  Especially if they can change their behaviour.
>
> It's hard to make a solid suggestion as to how your device is handled
> without knowing the intricacies of the device.

Yeah, I understand.
This one is a generic CPLD used in multiple switch models, however in this
switch model, it has the smallest set of features.
Things that are usable for the kernel and userspace it provides are SFP pins,
resets and debug information.
Debug information is quite detailed actually.

I have added the reset driver in v3 as that is something that was gonna get
added later as well as it exposes resets for the ethernet PHY-s and U-boot
messes with the OOB PHY configuration.

>
> Why do you require one single Regmap anyway?  Are they register banks
> not neatly separated on a per-function basis?

For GPIO and reset drivers, I could get away with each of them
registering a regmap
but the debug driver will require accessing certain registers from their space.
Also, I see using a single regmap that is provided by a generic driver
much simpler
and cleaner than doing that in each of the child drivers.

Regards,
Robert

>
> --
> Lee Jones [李琼斯]
> Senior Technical Lead - Developer Services
> Linaro.org │ Open source software for Arm SoCs
> Follow Linaro: Facebook | Twitter | Blog



-- 
Robert Marko
Staff Embedded Linux Engineer
Sartura Ltd.
Lendavska ulica 16a
10000 Zagreb, Croatia
Email: robert.marko@...tura.hr
Web: www.sartura.hr