auxdisplay: linedisp: Add support for changing scroll rate

If the message to display is longer than the number of characters that
the display can show, the message will be scrolled.  Currently the
scroll rate is fixed, moving every 500 ms.

Add support for changing the scroll rate through a "scroll_step_ms"
device attribute in sysfs.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

drivers/auxdisplay/line-display.c

@@ -19,6 +19,8 @@
 
 #include "line-display.h"
 
+#define DEFAULT_SCROLL_RATE	(HZ / 2)
+
 /**
  * linedisp_scroll() - scroll the display by a character
  * @t: really a pointer to the private data structure
@@ -50,7 +52,7 @@ static void linedisp_scroll(struct timer_list *t)
 	linedisp->scroll_pos %= linedisp->message_len;
 
 	/* rearm the timer */
-	if (linedisp->message_len > num_chars)
+	if (linedisp->message_len > num_chars && linedisp->scroll_rate)
 		mod_timer(&linedisp->timer, jiffies + linedisp->scroll_rate);
 }
 
@@ -149,8 +151,39 @@ static ssize_t message_store(struct device *dev, struct device_attribute *attr,
 
 static DEVICE_ATTR_RW(message);
 
+static ssize_t scroll_step_ms_show(struct device *dev,
+				   struct device_attribute *attr, char *buf)
+{
+	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+
+	return sysfs_emit(buf, "%u\n", jiffies_to_msecs(linedisp->scroll_rate));
+}
+
+static ssize_t scroll_step_ms_store(struct device *dev,
+				    struct device_attribute *attr,
+				    const char *buf, size_t count)
+{
+	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+	unsigned int ms;
+
+	if (kstrtouint(buf, 10, &ms) != 0)
+		return -EINVAL;
+
+	linedisp->scroll_rate = msecs_to_jiffies(ms);
+	if (linedisp->message && linedisp->message_len > linedisp->num_chars) {
+		del_timer_sync(&linedisp->timer);
+		if (linedisp->scroll_rate)
+			linedisp_scroll(&linedisp->timer);
+	}
+
+	return count;
+}
+
+static DEVICE_ATTR_RW(scroll_step_ms);
+
 static struct attribute *linedisp_attrs[] = {
 	&dev_attr_message.attr,
+	&dev_attr_scroll_step_ms.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(linedisp);
@@ -182,7 +215,7 @@ int linedisp_register(struct linedisp *linedisp, struct device *parent,
 	linedisp->update = update;
 	linedisp->buf = buf;
 	linedisp->num_chars = num_chars;
-	linedisp->scroll_rate = HZ / 2;
+	linedisp->scroll_rate = DEFAULT_SCROLL_RATE;
 
 	device_initialize(&linedisp->dev);
 	dev_set_name(&linedisp->dev, "linedisp.%lu",