auxdisplay: linedisp: add num_chars sysfs attribute

Add a read-only 'num_chars' sysfs attribute to report display digit count.

The num_chars attribute provides essential capability information to
userspace applications that need to know display dimensions before writing
messages, complementing the existing message and scroll controls.

No functional changes to existing behavior.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Author: jefflessard

Documentation/ABI/testing/sysfs-auxdisplay-linedisp

@@ -15,6 +15,17 @@ Description:
 		  echo "Hello World" > message
 		  cat message			# Returns "Hello World\n"
 
+What:		/sys/.../num_chars
+Date:		November 2025
+KernelVersion:	6.18
+Contact:	Jean-François Lessard <jefflessard3@gmail.com>
+Description:
+		Read-only attribute showing the character width capacity of
+		the line display device. Messages longer than this will scroll.
+
+		Example:
+		  cat num_chars		# Returns "16\n" for 16-char display
+
 What:		/sys/.../scroll_step_ms
 Date:		October 2021
 KernelVersion:	5.16

drivers/auxdisplay/line-display.c

@@ -178,6 +178,16 @@ static ssize_t message_store(struct device *dev, struct device_attribute *attr,
 
 static DEVICE_ATTR_RW(message);
 
+static ssize_t num_chars_show(struct device *dev, struct device_attribute *attr,
+			      char *buf)
+{
+	struct linedisp *linedisp = to_linedisp(dev);
+
+	return sysfs_emit(buf, "%u\n", linedisp->num_chars);
+}
+
+static DEVICE_ATTR_RO(num_chars);
+
 static ssize_t scroll_step_ms_show(struct device *dev,
 				   struct device_attribute *attr, char *buf)
 {
@@ -240,6 +250,7 @@ static DEVICE_ATTR(map_seg14, 0644, map_seg_show, map_seg_store);
 
 static struct attribute *linedisp_attrs[] = {
 	&dev_attr_message.attr,
+	&dev_attr_num_chars.attr,
 	&dev_attr_scroll_step_ms.attr,
 	&dev_attr_map_seg7.attr,
 	&dev_attr_map_seg14.attr,