linux-kernel - Re: [PATCH v1] pinctrl: mcp23s08: Reset all output latches to default at probe

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Message-ID: <CALHCpMj=-N5kToZ7kKbzrpeoMM=Ky+_=J=JnwDmBVRx1OPgxhg@mail.gmail.com>
Date: Mon, 6 Oct 2025 13:18:27 +0300
From: Maxim Kiselev <bigunclemax@...il.com>
To: Mike Looijmans <mike.looijmans@...ic.nl>
Cc: Linus Walleij <linus.walleij@...aro.org>, linux-gpio@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] pinctrl: mcp23s08: Reset all output latches to default
 at probe

Hi

пн, 6 окт. 2025 г. в 11:04, Mike Looijmans <mike.looijmans@...ic.nl>:
>
> On 06-10-2025 09:49, bigunclemax@...il.com wrote:
> > From: "Maksim Kiselev" <bigunclemax@...il.com>
> >
> > It appears that resetting only the direction register is not sufficient,
> > it's also necessary to reset the OLAT register to its default values.
> >
> > Otherwise, the following situation can occur:
> >
> > If a pin was configured as OUT=1 before driver probe(Ex: IODIR=1,IOLAT=1),
> > then after loading the MCP driver, the cache will be populated from
> > reg_defaults with IOLAT=0 (while the actual value in the chip is 1).
> > A subsequent setting OUT=0 will fail because
> > mcp_update_bits(mcp, MCP_OLAT, ...) calls regmap_update_bits(),
> > which will check that the value to be set (0) matches the cached value (0)
> > and thus skip writing actual value to the MCP chip.
>
> Maybe it's be better to fix the underlying issue: This driver should not be
> using a pre-populated regmap cache. Unless it performs a hard reset, the
> driver has no way of knowing what the initial values are, it should just read
> them from the chip.
>

I agree with you here, thought about it, but consider such a change
too radical :)

Okay, I'll remove the reg_defaults in the second version.

> >
> > To avoid this, the OLAT register must be explicitly reset at probe.
> >
> > Fixes: 3ede3f8b4b4b ("pinctrl: mcp23s08: Reset all pins to input at probe")
> > Signed-off-by: Maksim Kiselev <bigunclemax@...il.com>
> > ---
> >   drivers/pinctrl/pinctrl-mcp23s08.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/drivers/pinctrl/pinctrl-mcp23s08.c b/drivers/pinctrl/pinctrl-mcp23s08.c
> > index 78ff7930649d..23af441aa468 100644
> > --- a/drivers/pinctrl/pinctrl-mcp23s08.c
> > +++ b/drivers/pinctrl/pinctrl-mcp23s08.c
> > @@ -622,6 +622,11 @@ int mcp23s08_probe_one(struct mcp23s08 *mcp, struct device *dev,
> >       if (ret < 0)
> >               return ret;
> >
> > +     /* Also reset all out latches to default values */
> > +     ret = mcp_write(mcp, MCP_OLAT, 0x0);
> > +     if (ret < 0)
> > +             return ret;
> > +
> >       /* verify MCP_IOCON.SEQOP = 0, so sequential reads work,
> >        * and MCP_IOCON.HAEN = 1, so we work with all chips.
> >        */
>
> Mike.
>
>
>