platform/x86: serial-multi-instantiate: Don't require both I2C and SPI

Change the Kconfig dependency so that it doesn't require both I2C and SPI
subsystems to be built. Make a few small changes to the code so that the
code for a bus is only called if the bus is being built.

When SPI support was added to serial-multi-instantiate it created a
dependency that both CONFIG_I2C and CONFIG_SPI must be enabled.
Typically they are, but there's no reason why this should be a
requirement. A specific kernel build could have only I2C devices
or only SPI devices. It should be possible to use serial-multi-instantiate
if only I2C or only SPI is enabled.

The dependency formula used is:

  depends on (I2C && !SPI) || (!I2C && SPI) || (I2C && SPI)

The advantage of this approach is that if I2C=m or SPI=m then
SERIAL_MULTI_INSTANTIATE is limited to n/m.
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20240814132939.308696-1-rf@opensource.cirrus.comReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/x86/Kconfig

@@ -1000,7 +1000,8 @@ config TOPSTAR_LAPTOP
 
 config SERIAL_MULTI_INSTANTIATE
 	tristate "Serial bus multi instantiate pseudo device driver"
-	depends on I2C && SPI && ACPI
+	depends on ACPI
+	depends on (I2C && !SPI) || (!I2C && SPI) || (I2C && SPI)
 	help
 	  Some ACPI-based systems list multiple devices in a single ACPI
 	  firmware-node. This driver will instantiate separate clients

drivers/platform/x86/serial-multi-instantiate.c

@@ -83,11 +83,15 @@ static int smi_get_irq(struct platform_device *pdev, struct acpi_device *adev,
 
 static void smi_devs_unregister(struct smi *smi)
 {
+#if IS_REACHABLE(CONFIG_I2C)
 	while (smi->i2c_num--)
 		i2c_unregister_device(smi->i2c_devs[smi->i2c_num]);
+#endif
 
+	if (IS_REACHABLE(CONFIG_SPI)) {
 		while (smi->spi_num--)
 			spi_unregister_device(smi->spi_devs[smi->spi_num]);
+	}
 }
 
 /**
@@ -258,9 +262,15 @@ static int smi_probe(struct platform_device *pdev)
 
 	switch (node->bus_type) {
 	case SMI_I2C:
+		if (IS_REACHABLE(CONFIG_I2C))
 			return smi_i2c_probe(pdev, smi, node->instances);
+
+		return -ENODEV;
 	case SMI_SPI:
+		if (IS_REACHABLE(CONFIG_SPI))
 			return smi_spi_probe(pdev, smi, node->instances);
+
+		return -ENODEV;
 	case SMI_AUTO_DETECT:
 		/*
 		 * For backwards-compatibility with the existing nodes I2C
@@ -270,10 +280,16 @@ static int smi_probe(struct platform_device *pdev)
 		 * SpiSerialBus nodes that were previously ignored, and this
 		 * preserves that behavior.
 		 */
+		if (IS_REACHABLE(CONFIG_I2C)) {
 			ret = smi_i2c_probe(pdev, smi, node->instances);
 			if (ret != -ENOENT)
 				return ret;
+		}
+
+		if (IS_REACHABLE(CONFIG_SPI))
 			return smi_spi_probe(pdev, smi, node->instances);
+
+		return -ENODEV;
 	default:
 		return -EINVAL;
 	}