linux-kernel - Re: [PATCH v2 0/3] pinctrl: single: bit-per-mux DT flexibility, probe robustness, and consistent pinconf offsets

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Message-ID:
 <OSQPR06MB7252ADEF2CB075BC40AD7DB68B66A@OSQPR06MB7252.apcprd06.prod.outlook.com>
Date: Fri, 6 Feb 2026 07:24:12 +0000
From: Billy Tsai <billy_tsai@...eedtech.com>
To: Tony Lindgren <tony@...mide.com>
CC: Haojian Zhuang <haojian.zhuang@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-omap@...r.kernel.org"
	<linux-omap@...r.kernel.org>, "linux-gpio@...r.kernel.org"
	<linux-gpio@...r.kernel.org>, Linus Walleij <linusw@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"andrew@...econstruct.com.au" <andrew@...econstruct.com.au>, BMC-SW
	<BMC-SW@...eedtech.com>
Subject: Re: [PATCH v2 0/3] pinctrl: single: bit-per-mux DT flexibility, probe
 robustness, and consistent pinconf offsets

* Billy Tsai <billy_tsai@...eedtech.com> [260204 06:54]:
> > Hi Tony,
> >
> > This series proposes a set of changes to pinctrl-single motivated by
> > bit-per-mux SoC designs such as ASPEED AST2700 (per-pin DT encoding,
> > aligned pinconf offsets, and allowing probe to continue when the MMIO
> > region is already reserved).
> >
> > Linus reviewed the series and noted that he would prefer a custom
> > pinctrl driver using existing helpers and the pinmux = <...> DT
> > property, rather than extending pinctrl-single, and suggested that the
> > pinctrl-single maintainers review the approach before any merge
> > decision.
> >
> > I would appreciate your guidance on whether extending
> > pinctrl-single in this direction is acceptable, or if the preference is
> > to pursue a dedicated driver instead.

> I agree with what Linus that separate more targeted drivers are better
> to avoid the drivers getting complex. With the GENERIC_PIN* helpers doing
> targeted drivers should be trivial.

> My preference would be to move the bit-per-mux handling out of the
> pinctrl-single driver into a separate pinctrl-single-bit type driver.
> Seems that can still handle the cases where no hardware specific driver
> is needed.

> This would simplify pinctrl-single driver quite a bit, and would make
> the new driver quite simple too AFAIK.

Hi Tony,

Thanks for the clarification.

I understand the preference is to keep pinctrl-single minimal and move
the bit-per-mux handling into a separate, more targeted driver built on
top of the GENERIC_PINMUX/GENERIC_PINCONF helpers, rather than extending
pinctrl-single itself.

Based on that, I’ll look into refactoring this into a
pinctrl-single-bit style driver that covers bit-per-mux / bit-per-pin
layouts generically (including AST2700), while keeping pinctrl-single
focused on the simpler register models.

One additional point I’d like to raise is the handling of pre-reserved
MMIO regions.

On AST2700 systems, the SCU register range containing the pinctrl
registers is commonly reserved by a top-level syscon node or by firmware.
In this setup, devm_request_mem_region() can return -EBUSY even though the
registers are valid and intended to be shared, which currently causes the
driver to fail probing and leaves pinmux unconfigured.

When moving to a separate targeted driver, would the preferred approach
be to treat this condition as a warning and continue probing, or is there
an alternative pattern you’d recommend for handling shared SCU-style
register blocks in pinctrl drivers?

Thanks,
Billy