Re: [PATCH v11 03/10] spi: Add multi-cs memories support in SPI core

[Guenter Roeck <linux@xxxxxxxxxxxx>]

On 1/21/24 10:06, Michael Walle wrote:
> > > > FWIW, the problem is due to
> > >
> > > > +#define SPI_CS_CNT_MAX 4
> > >
> > > > in the offending patch, but apeed2400 FMC supports up to 5 SPI chip selects.
> > > >
> > > >   static const struct aspeed_spi_data ast2400_fmc_data = {
> > > >          .max_cs        = 5,
> > > >     ^^^^^^^^^^^^^^^^^^^
> > > >          .hastype       = true,
> > >
> > > > Limiting .max_cs to 4 or increasing SPI_CS_CNT_MAX to 5 fixes the problem,
> > > > though of course I don't know if increasing SPI_CS_CNT_MAX has other side
> > > > effects.
> > >
> > > Yeah, I was coming to a similar conclusion myself - the limit is just
> > > too low.  I can't see any problem with increasing it.
> >
> > It would cost a bit of memory and somewhat affect performance sine many
> > of the newly introduced loops are bound by SPI_CS_CNT_MAX and not by
> > num_chipselect.
> >
> > It also might make sense to document the new limit somewhere. Prior
> > to this commit it was not limited at all.
> > Documentation/devicetree/bindings/spi/spi-davinci.txt lists 5 chip
> > selects in its example for the use of cs-gpios.
> > Documentation/devicetree/bindings/spi/spi-controller.yaml also does not
> > list a limit.
>
> Given that, that the rest of this series is under discussion (and esp. whether
> it is the correct way to do it) it might make sense to just revert the picked
> patches.

I can't really comment on that, but I found that there is another
affected devicetree property: num-cs. Its range isn't limited in
Documentation/devicetree/bindings/spi/spi-controller.yaml. Various
dts/dtsi files use numbers as large as 8. The range is limited in some
bindings files, but not all of them.
Documentation/devicetree/bindings/spi/spi-lantiq-ssc.txt, for example,
says

  "num-cs: see spi-bus.txt, set to 8 if unset"

Various Broadcom dtsi files set it to 8.

So I guess 8 would be the absolute minimum to re-enable support for
affected systems.

Guenter