platform/x86: thinkpad_acpi: sysfs interface to auxmac

Newer Thinkpads have a feature called MAC Address Pass-through.
This patch provides a sysfs interface that userspace can use
to get this auxiliary mac address.
Signed-off-by: Fernando Eckhardt Valle <fevalle@ipt.br>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230926202144.5906-1-fevalle@ipt.brReviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

Documentation/admin-guide/laptops/thinkpad-acpi.rst

@@ -53,6 +53,7 @@ detailed description):
 	- Lap mode sensor
 	- Setting keyboard language
 	- WWAN Antenna type
+	- Auxmac
 
 A compatibility table by model and feature is maintained on the web
 site, http://ibm-acpi.sf.net/. I appreciate any success or failure
@@ -1511,6 +1512,25 @@ Currently 2 antenna types are supported as mentioned below:
 The property is read-only. If the platform doesn't have support the sysfs
 class is not created.
 
+Auxmac
+------
+
+sysfs: auxmac
+
+Some newer Thinkpads have a feature called MAC Address Pass-through. This
+feature is implemented by the system firmware to provide a system unique MAC,
+that can override a dock or USB ethernet dongle MAC, when connected to a
+network. This property enables user-space to easily determine the MAC address
+if the feature is enabled.
+
+The values of this auxiliary MAC are:
+
+        cat /sys/devices/platform/thinkpad_acpi/auxmac
+
+If the feature is disabled, the value will be 'disabled'.
+
+This property is read-only.
+
 Adaptive keyboard
 -----------------
 

drivers/platform/x86/thinkpad_acpi.c

@@ -10785,6 +10785,89 @@ static struct ibm_struct dprc_driver_data = {
 	.name = "dprc",
 };
 
+/*
+ * Auxmac
+ *
+ * This auxiliary mac address is enabled in the bios through the
+ * MAC Address Pass-through feature. In most cases, there are three
+ * possibilities: Internal Mac, Second Mac, and disabled.
+ *
+ */
+
+#define AUXMAC_LEN 12
+#define AUXMAC_START 9
+#define AUXMAC_STRLEN 22
+#define AUXMAC_BEGIN_MARKER 8
+#define AUXMAC_END_MARKER 21
+
+static char auxmac[AUXMAC_LEN + 1];
+
+static int auxmac_init(struct ibm_init_struct *iibm)
+{
+	acpi_status status;
+	struct acpi_buffer buffer = { ACPI_ALLOCATE_BUFFER, NULL };
+	union acpi_object *obj;
+
+	status = acpi_evaluate_object(NULL, "\\MACA", NULL, &buffer);
+
+	if (ACPI_FAILURE(status))
+		return -ENODEV;
+
+	obj = buffer.pointer;
+
+	if (obj->type != ACPI_TYPE_STRING || obj->string.length != AUXMAC_STRLEN) {
+		pr_info("Invalid buffer for MAC address pass-through.\n");
+		goto auxmacinvalid;
+	}
+
+	if (obj->string.pointer[AUXMAC_BEGIN_MARKER] != '#' ||
+	    obj->string.pointer[AUXMAC_END_MARKER] != '#') {
+		pr_info("Invalid header for MAC address pass-through.\n");
+		goto auxmacinvalid;
+	}
+
+	if (strncmp(obj->string.pointer + AUXMAC_START, "XXXXXXXXXXXX", AUXMAC_LEN) != 0)
+		strscpy(auxmac, obj->string.pointer + AUXMAC_START, sizeof(auxmac));
+	else
+		strscpy(auxmac, "disabled", sizeof(auxmac));
+
+free:
+	kfree(obj);
+	return 0;
+
+auxmacinvalid:
+	strscpy(auxmac, "unavailable", sizeof(auxmac));
+	goto free;
+}
+
+static struct ibm_struct auxmac_data = {
+	.name = "auxmac",
+};
+
+static ssize_t auxmac_show(struct device *dev,
+			   struct device_attribute *attr,
+			   char *buf)
+{
+	return sysfs_emit(buf, "%s\n", auxmac);
+}
+static DEVICE_ATTR_RO(auxmac);
+
+static umode_t auxmac_attr_is_visible(struct kobject *kobj,
+				      struct attribute *attr, int n)
+{
+	return auxmac[0] == 0 ? 0 : attr->mode;
+}
+
+static struct attribute *auxmac_attributes[] = {
+	&dev_attr_auxmac.attr,
+	NULL
+};
+
+static const struct attribute_group auxmac_attr_group = {
+	.is_visible = auxmac_attr_is_visible,
+	.attrs = auxmac_attributes,
+};
+
 /* --------------------------------------------------------------------- */
 
 static struct attribute *tpacpi_driver_attributes[] = {
@@ -10843,6 +10926,7 @@ static const struct attribute_group *tpacpi_groups[] = {
 	&proxsensor_attr_group,
 	&kbdlang_attr_group,
 	&dprc_attr_group,
+	&auxmac_attr_group,
 	NULL,
 };
 
@@ -11414,6 +11498,10 @@ static struct ibm_init_struct ibms_init[] __initdata = {
 		.init = tpacpi_dprc_init,
 		.data = &dprc_driver_data,
 	},
+	{
+		.init = auxmac_init,
+		.data = &auxmac_data,
+	},
 };
 
 static int __init set_ibm_param(const char *val, const struct kernel_param *kp)