linux-kernel - Re: [PATCH v6 5/6] dt-bindings: mfd: Add Delta TN48M CPLD drivers bindings

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Date:   Wed, 11 Aug 2021 14:17:45 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Robert Marko <robert.marko@...tura.hr>
Cc:     Rob Herring <robh@...nel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Lee Jones <lee.jones@...aro.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <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 v6 5/6] dt-bindings: mfd: Add Delta TN48M CPLD drivers bindings

On Tue, Aug 3, 2021 at 9:23 PM Robert Marko <robert.marko@...tura.hr> wrote:

> The pins that this driver wants to expose are used for SFP-s only,
> they are provided by the Lattice CPLD which also does other things.
>
> Linux has a generic SFP driver which is used to manage these SFP
> ports, but it only supports GPIO-s, it has no concept of anything else.
> Since the driver is fully generic, I have no idea how could one extend it
> to effectively handle these pins internally, especially since I have more
> switches that use the CPLD for SFP-s as well, even for 48 ports and 192
> pins for them.

Which file is this driver in so I can look?

Maybe it is not a good idea to look for generic code just because
it is convenient? I have had this problem before with GPIO, along
the lines "lemme just do this dirty thing this one time because it
is so convenient for me" (more or less) and the answer is still
"no".

Can you either augment the driver to handle a regmap with bit indices
instead or write a new similar driver for that or refactor it some other
way?

It is not a simple solution to your problem, but it might be the right
solution even if it means some more work.

> GPIO regmap works perfectly for this as its generic enough to cover all of
> these cases.

Yeah but it might be the wrong thing to do even if it is simple
to use and works.

> CPLD also provides pins to test the port LED-s per color as well,
> but I have chosen not to expose them so far.

Have you considered
Documentation/devicetree/bindings/leds/register-bit-led.txt

> > If it is a regmap in Linux then that is fine, just pass the regmap
> > around inside the kernel, OK finished. But really that is an OS
> > detail.
>
> Yes, its regmap but I cant really pass it to the SFP driver as I don't have
> special driver handling the SFP but rather the generic kernel one.
> It only knows how to handle GPIO-s.

Of course you have to program it. If I know which driver it
is it is easier to provide architecture ideas.

Yours,
Linus Walleij