Re: [PATCH 2/2] dt-bindings: pinctrl: mcp23s08: Add multi-chip example

[Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org>]

Hi Linus,

Le lundi 19 janvier 2026, 00:12:05 heure normale d’Europe centrale Linus 
Walleij a écrit :
> On Wed, Jan 14, 2026 at 6:29 PM Jean-Michel Hautbois
> 
> <jeanmichel.hautbois@yoseli.org> wrote:
> > > Make some fancy - if: clauses to decide the maxItems from the
> > > compatible.
> > > Don't hesitate to ask for help if this gets complicated, I get a panic
> > > every time I have to deal with it because of the whitespacing business.
> > 
> > Indeed, I was probably a bit optimistic :-).
> > 
> > What about this:
> >  allOf:
> >    - $ref: /schemas/spi/spi-peripheral-props.yaml#
> > 
> > +  - if:
> > +      properties:
> > +        compatible:
> > +          contains:
> > +            enum:
> > +              - microchip,mcp23s08
> > +              - microchip,mcp23008
> > +    then:
> > +      properties:
> > +        gpio-line-names:
> > +          maxItems: 32
> > +        pinmux:
> > +          properties:
> > +            pins:
> > +              maxItems: 32
> > +  - if:
> > +      properties:
> > +        compatible:
> > +          contains:
> > +            enum:
> > +              - microchip,mcp23s17
> > +              - microchip,mcp23s18
> > +              - microchip,mcp23017
> > +              - microchip,mcp23018
> > +    then:
> > +      properties:
> > +        gpio-line-names:
> > +          maxItems: 128
> > +        pinmux:
> > +          properties:
> > +            pins:
> > +              maxItems: 128
> > +
> > 
> > This would allow up to 4 chips to be on the same CS.
> 
> Hm I don't get it why everything is multiplied by 4 here?
> 
> Doesn't each chip have its own instance? And the instance is
> limited to 8 or 16 lines/pins?
> 
> > But in the
> > microchip,mcp23s17 datasheet, it says:
> > "Three Hardware Address Pins to Allow Up to Eight Devices On the Bus"
> 
> Doesn't each device have its own entry?

As far as I get it, for I2C variants, each chip has its own device node,
so gpio-line-names would be limited to 8/16 per node.

However, for SPI variants (mcp23s08/s17/s18), multiple chips can share
the same SPI chipselect via the 'microchip,spi-present-mask' property.

In this case, a single DT node represents multiple physical chips,
and gpio-line-names needs to cover all of them:
- mcp23s08: up to 4 chips × 8 GPIOs = 32 lines
- mcp23s17: up to 8 chips × 16 GPIOs = 128 lines

Should I split the conditionals to apply different limits for I2C vs SPI?

Or did I miss something which is totally possible too -_-' ?

Thanks,
JM

> Yours,
> Linus Walleij