platform/x86: asus-wmi: Add support for MCU powersave

Add support for an MCU powersave WMI call. This is intended to set the
MCU in to a low-power mode when sleeping. This mode can cut sleep power
use by around half.
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Luke D. Jones <luke@ljones.dev>
Link: https://lore.kernel.org/r/20240404001652.86207-9-luke@ljones.devSigned-off-by: Hans de Goede <hdegoede@redhat.com>

Documentation/ABI/testing/sysfs-platform-asus-wmi

@@ -203,3 +203,12 @@ Description:
 		Set if the BIOS POST sound is played on boot.
 			* 0 - False,
 			* 1 - True
+
+What:		/sys/devices/platform/<platform>/mcu_powersave
+Date:		Apr 2024
+KernelVersion:	6.10
+Contact:	"Luke Jones" <luke@ljones.dev>
+Description:
+		Set if the MCU can go in to low-power mode on system sleep
+			* 0 - False,
+			* 1 - True

drivers/platform/x86/asus-wmi.c

@@ -1303,6 +1303,53 @@ static ssize_t nv_temp_target_show(struct device *dev,
 }
 static DEVICE_ATTR_RW(nv_temp_target);
 
+/* Ally MCU Powersave ********************************************************/
+static ssize_t mcu_powersave_show(struct device *dev,
+				   struct device_attribute *attr, char *buf)
+{
+	struct asus_wmi *asus = dev_get_drvdata(dev);
+	int result;
+
+	result = asus_wmi_get_devstate_simple(asus, ASUS_WMI_DEVID_MCU_POWERSAVE);
+	if (result < 0)
+		return result;
+
+	return sysfs_emit(buf, "%d\n", result);
+}
+
+static ssize_t mcu_powersave_store(struct device *dev,
+				    struct device_attribute *attr,
+				    const char *buf, size_t count)
+{
+	int result, err;
+	u32 enable;
+
+	struct asus_wmi *asus = dev_get_drvdata(dev);
+
+	result = kstrtou32(buf, 10, &enable);
+	if (result)
+		return result;
+
+	if (enable > 1)
+		return -EINVAL;
+
+	err = asus_wmi_set_devstate(ASUS_WMI_DEVID_MCU_POWERSAVE, enable, &result);
+	if (err) {
+		pr_warn("Failed to set MCU powersave: %d\n", err);
+		return err;
+	}
+
+	if (result > 1) {
+		pr_warn("Failed to set MCU powersave (result): 0x%x\n", result);
+		return -EIO;
+	}
+
+	sysfs_notify(&asus->platform_device->dev.kobj, NULL, "mcu_powersave");
+
+	return count;
+}
+static DEVICE_ATTR_RW(mcu_powersave);
+
 /* Battery ********************************************************************/
 
 /* The battery maximum charging percentage */
@@ -4317,6 +4364,7 @@ static struct attribute *platform_attributes[] = {
 	&dev_attr_ppt_platform_sppt.attr,
 	&dev_attr_nv_dynamic_boost.attr,
 	&dev_attr_nv_temp_target.attr,
+	&dev_attr_mcu_powersave.attr,
 	&dev_attr_boot_sound.attr,
 	&dev_attr_panel_od.attr,
 	&dev_attr_mini_led_mode.attr,
@@ -4370,6 +4418,8 @@ static umode_t asus_sysfs_is_visible(struct kobject *kobj,
 		devid = ASUS_WMI_DEVID_NV_DYN_BOOST;
 	else if (attr == &dev_attr_nv_temp_target.attr)
 		devid = ASUS_WMI_DEVID_NV_THERM_TARGET;
+	else if (attr == &dev_attr_mcu_powersave.attr)
+		devid = ASUS_WMI_DEVID_MCU_POWERSAVE;
 	else if (attr == &dev_attr_boot_sound.attr)
 		devid = ASUS_WMI_DEVID_BOOT_SOUND;
 	else if (attr == &dev_attr_panel_od.attr)