Re: [PATCH v2 2/3] platform/x86: serial-multi-instantiate: Fix SPI chip select on platforms with incomplete ACPI cs-gpios

[Richard Fitzgerald <rf@opensource.cirrus.com>]

On 13/04/2026 12:43 pm, Hans de Goede wrote:
> Hi,
> 
> On 13-Apr-26 12:05 PM, Khalil wrote:
>> Some HP laptops with Intel Lunar Lake and dual Cirrus Logic CS35L56
>> amplifiers over SPI have an incomplete cs-gpios property in the SPI
>> controller's _DSD - it only declares the first chip select. The
>> remaining chip select GPIOs are defined as GpioIo resources in the
>> peripheral's ACPI node but are not referenced by the controller.
>>
>> This causes the SPI framework to reject devices whose chip select
>> exceeds num_chipselect with -EINVAL, preventing the second amplifier
>> from probing.
>>
>> Fix this on known affected platforms by:
>>

<SNIP>

>> +
>> +/*
>> + * ACPI GPIO mapping for the chip select GpioIo resource.
>> + * Maps "cs-gpios" to the GpioIo resource at index 0 of the ACPI _CRS.
>> + */
>> +static const struct acpi_gpio_params smi_cs_gpio_params = { 0, 0, false };
>> +
>> +static const struct acpi_gpio_mapping smi_cs_gpio_mapping[] = {
>> +	{ "cs-gpios", &smi_cs_gpio_params, 1 },
>> +	{ }
>>   };
> 
> I'm confused here you say that the chip-select for the second amplifier
> is missing, but I would expect that to be the GpioIo resource at index 1
> not 0?
> 
> I would expect the GpioIo resource at index 0 to be for the first
> amplifier which does have a GPIO already in the _DSD cs-gpios property ?
> 
> Since this is a multi-serial device ACPI node, there is only one
> set of resources for both amplifiers (with their also being 2 SPI
> resources in the _CRS list).
> 
> So I would expect there to also be multiple GPIO entries in the
> _CRS list, with the order of the chip-selects GPIOs resources matching
> the order of the SPI resources.

You can't assume that. There is no ACPI reason why the GpioIo should be
in order of chip selects. In fact, some manufacturers scramble them
(e.g. CS2, CS1, CS3). You have to remember that the ACPI is written so
that it works with Windows (which is the reason we need this serial
multi-instantiate driver.) so it may contain Windowsisms.