auxdisplay: linedisp: display static message when length <= display size

Currently, when a message shorter than the display size is written, the
content wraps around (e.g., "123" on a 4-digit display shows "1231")
without scrolling, which is confusing and unintuitive.

Change behavior to display short messages statically with space padding
(e.g. "123 ") while only scrolling messages longer than the display width.
This provides more natural behavior that aligns with user expectations
and current linedisp_display() kernel-doc.

The scroll logic is also consolidated into a helper function for clarity.

No API changes are introduced.

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

Author: jefflessard

drivers/auxdisplay/line-display.c

@@ -36,6 +36,11 @@ static struct linedisp *to_linedisp(struct device *dev)
 	return container_of(dev, struct linedisp, dev);
 }
 
+static inline bool should_scroll(struct linedisp *linedisp)
+{
+	return linedisp->message_len > linedisp->num_chars && linedisp->scroll_rate;
+}
+
 /**
  * linedisp_scroll() - scroll the display by a character
  * @t: really a pointer to the private data structure
@@ -67,8 +72,7 @@ static void linedisp_scroll(struct timer_list *t)
 	linedisp->scroll_pos %= linedisp->message_len;
 
 	/* rearm the timer */
-	if (linedisp->message_len > num_chars && linedisp->scroll_rate)
-		mod_timer(&linedisp->timer, jiffies + linedisp->scroll_rate);
+	mod_timer(&linedisp->timer, jiffies + linedisp->scroll_rate);
 }
 
 /**
@@ -118,8 +122,16 @@ static int linedisp_display(struct linedisp *linedisp, const char *msg,
 	linedisp->message_len = count;
 	linedisp->scroll_pos = 0;
 
-	/* update the display */
-	linedisp_scroll(&linedisp->timer);
+	if (should_scroll(linedisp)) {
+		/* display scrolling message */
+		linedisp_scroll(&linedisp->timer);
+	} else {
+		/* display static message */
+		memset(linedisp->buf, ' ', linedisp->num_chars);
+		memcpy(linedisp->buf, linedisp->message,
+		       umin(linedisp->num_chars, linedisp->message_len));
+		linedisp->ops->update(linedisp);
+	}
 
 	return 0;
 }
@@ -186,12 +198,12 @@ static ssize_t scroll_step_ms_store(struct device *dev,
 	if (err)
 		return err;
 
+	timer_delete_sync(&linedisp->timer);
+
 	linedisp->scroll_rate = msecs_to_jiffies(ms);
-	if (linedisp->message && linedisp->message_len > linedisp->num_chars) {
-		timer_delete_sync(&linedisp->timer);
-		if (linedisp->scroll_rate)
-			linedisp_scroll(&linedisp->timer);
-	}
+
+	if (should_scroll(linedisp))
+		linedisp_scroll(&linedisp->timer);
 
 	return count;
 }