linux-kernel - Re: [PATCH 0/2] pinctrl: zynqmp: Support muxing individual pins

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Date: Mon, 6 May 2024 08:43:49 +0200
From: Linus Walleij <linus.walleij@...aro.org>
To: Sean Anderson <sean.anderson@...ux.dev>
Cc: Michal Simek <michal.simek@....com>, linux-gpio@...r.kernel.org, 
	Krishna Potthuri <sai.krishna.potthuri@....com>, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, Conor Dooley <conor+dt@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Rob Herring <robh@...nel.org>, devicetree@...r.kernel.org
Subject: Re: [PATCH 0/2] pinctrl: zynqmp: Support muxing individual pins

On Fri, May 3, 2024 at 6:22 PM Sean Anderson <sean.anderson@...ux.dev> wrote:

> This series adds support for muxing individual pins, instead of
> requiring groups to be muxed together. See [1] for additional
> discussion.
>
> [1] https://lore.kernel.org/linux-arm-kernel/5bb0dc7e-4c89-4f3d-abc6-41ae9ded5ae9@linux.dev/

The way I usually would recommend to solve this would be to
define new subgroups, so e.g. for a UARTS:

uart0_grp = pin_rx, pin_tx, pin_cts, pin_dts, pin_dcd;

And today this would be used like that:

mux0:
    function = "uart0";
    groups = "uart0_grp";

Then we realize that not everyone need all the modem
control signals provided. What to do. Well this:

uart0_rxtx_grp = pin_rx, pin_tx:
uart0_modem_grp = pin_cts, pin_dts, pin_dcd;

mux0:
    function = "uart0";
    groups = "uart0_rxtx_grp";

Now the CTS, DTS, DCD pins can be reused for something
else such as GPIO.

I *know* that this breaks ABI: the driver group definitions change
and the device tree needs to be changed too.

This only matters if the users have a habit of distributing the
kernel and DTB separately so a new kernel needs to support
and old DTB. This varies in how much control we have but I
think for Xilinx systems the kernel and DTB are always updated
in lockstep, so it really does not matter?

Yours,
Linus Walleij