mlxbf-bootctl: Support sysfs entries for RTC battery status

This patch extends the mlxbf-bootctl driver's sysfs entries
to support read access for the board's RTC battery status.
A successful read from this attribute returns the status of
the board's RTC battery. The RTC battery status register is
also cleared upon successful read operation.

Signed-off-by: Xiangrong Li <[e-mail]>
Reviewed-by: David Thompson <[e-mail]>
Reviewed-by: Ilpo Järvinen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ilpo Järvinen <[e-mail]>

Documentation/ABI/testing/sysfs-platform-mellanox-bootctl

···
 		The "mfg_lock" sysfs attribute is write-only.
                 A successful write to this attribute will latch the
                 board-level attributes into EEPROM, making them read-only.
+
+What:		/sys/bus/platform/devices/MLNXBF04:00/rtc_battery
+Date:		June 2025
+KernelVersion:	6.15
+Contact:	"Xiangrong Li <[e-mail]>"
+Description:
+		The "rtc_battery" sysfs attribute is read-only.
+		A successful read from this attribute returns the status of
+		the board's RTC battery. The RTC battery status register is
+		also cleared upon successful read operation.

drivers/platform/mellanox/mlxbf-bootctl.c

···
 static DEFINE_MUTEX(icm_ops_lock);
 static DEFINE_MUTEX(os_up_lock);
 static DEFINE_MUTEX(mfg_ops_lock);
+static DEFINE_MUTEX(rtc_ops_lock);
 
 /*
  * Objects are stored within the MFG partition per type.
···
 	mutex_unlock(&icm_ops_lock);
 
 	return res.a0 ? -EPERM : count;
+}
+
+static ssize_t rtc_battery_show(struct device *dev,
+				struct device_attribute *attr,
+				char *buf)
+{
+	struct arm_smccc_res res;
+
+	mutex_lock(&rtc_ops_lock);
+	arm_smccc_smc(MLNX_HANDLE_GET_RTC_LOW_BATT, 0, 0, 0, 0,
+		      0, 0, 0, &res);
+	mutex_unlock(&rtc_ops_lock);
+
+	if (res.a0)
+		return -EPERM;
+
+	return sysfs_emit(buf, "0x%lx\n", res.a1);
 }
 
 static ssize_t os_up_store(struct device *dev,
···
 static DEVICE_ATTR_RW(uuid);
 static DEVICE_ATTR_RW(rev);
 static DEVICE_ATTR_WO(mfg_lock);
+static DEVICE_ATTR_RO(rtc_battery);
 
 static struct attribute *mlxbf_bootctl_attrs[] = {
 	&dev_attr_post_reset_wdog.attr,
···
 	&dev_attr_uuid.attr,
 	&dev_attr_rev.attr,
 	&dev_attr_mfg_lock.attr,
+	&dev_attr_rtc_battery.attr,
 	NULL
 };
 

drivers/platform/mellanox/mlxbf-bootctl.h

···
  */
 #define MLNX_HANDLE_OS_UP    0x82000014
 
+/*
+ * SMC function ID to get and clear the RTC low voltage bit
+ */
+#define MLNX_HANDLE_GET_RTC_LOW_BATT   0x82000023
+
 /* SMC function IDs for SiP Service queries */
 #define MLXBF_BOOTCTL_SIP_SVC_CALL_COUNT	0x8200ff00
 #define MLXBF_BOOTCTL_SIP_SVC_UID		0x8200ff01