Re: (EXT) Re: [PATCH 1/1] spi: lpspi: Add support for 'num-cs' property

[Alexander Stein <alexander.stein@xxxxxxxxxxxxxxx>]

Am Dienstag, dem 07.12.2021 um 12:57 +0000 schrieb Mark Brown:
> * PGP Signed by an unknown key
> 
> On Tue, Dec 07, 2021 at 11:41:14AM +0100, Alexander Stein wrote:
> 
> > +	if (!device_property_read_u32(&pdev->dev, "num-cs", &num_cs))
> > +		controller->num_chipselect = num_cs;
> > +	else
> > +		controller->num_chipselect = 1;
> 
> Do we need to use the num_cs property or can we just set num_chipselect
> to whatever the maximum value that can be set is?  I've never been 100%
> clear on why num-cs is in the binding.

I see two things which needs to be considered when setting
num_chipselect:

1.
The hardware limitation in the uC. The i.MX8XQP RM says:
>  The entire CS field is not fully supported in every LPSPI module
> instance. Refer to the chip-specific information for LPSPI.

This indicates there are some i.MX, not necessarily i.MX8 only, which
have more than 2 hardware chip selects. This might be set depending on
the compatible.

2.
The hardware limitation on the SOM and/or mainboard. E.g. even if the
uC supports 2 chip selects, only one may be available on the board
connector. This is something which can only be set in the DT.
This case is what this patch is about: Providing 2 hardware chip
selects, as the default (if unset) is just one.

Best regards,
Alexander