--- a/backlight.1	Wed Oct 09 16:54:50 2019 +0200
+++ b/backlight.1	Fri Oct 11 12:58:48 2019 +0200
@@ -23,9 +23,14 @@
 .\" SYNOPSIS
 .Sh SYNOPSIS
 .Nm
-.Ar down
+.Ar decrease
+.Nm
+.Ar get
 .Nm
-.Ar up
+.Ar increase
+.Nm
+.Ar set
+.Ar percentage
 .\" DESCRIPTION
 .Sh DESCRIPTION
 The
@@ -33,11 +38,19 @@
 tool adjust the brightness on your screen using direct access to ACPI video
 extension.
 .Pp
-It understand the two commands
-.Ar down
-and
-.Ar up
-which decrease or increase the brightness level respectively.
+The following commands are available:
+.Bl -tag -width 10n
+.It Cm decrease
+Decrease to the next lower level.
+.It Cm get
+Get the current level scaled to a percentage range.
+.It Cm increase
+Increase to the next upper level.
+.It Cm set
+Set brightness to the
+.Ar percentage
+value.
+.El
 .\" AUTHORS
 .Sh AUTHORS
 .Nm

--- a/backlight.c	Wed Oct 09 16:54:50 2019 +0200
+++ b/backlight.c	Fri Oct 11 12:58:48 2019 +0200
@@ -22,17 +22,35 @@
 #include <stdlib.h>
 #include <string.h>
 
-enum action {
-	None,
-	Up,
-	Down
-};
+/*
+ * Functions to implement for your system:
+ *
+ * All of these functions are allowed to exit on failures.
+ */
+
+/* Increase to the nearest upper level */
+static void
+increase(void);
+
+/* Decrease to the nearest lower level */
+static void
+decrease(void);
+
+/* Set the level explicitly to the nearest value */
+static void
+set(unsigned int);
+
+/* Get the current value */
+static unsigned int
+get(void);
 
 static void
 usage(const char *name)
 {
-	fprintf(stderr, "usage: %s down\n", name);
-	fprintf(stderr, "       %s up\n", name);
+	fprintf(stderr, "usage: %s decrease\n", name);
+	fprintf(stderr, "       %s get\n", name);
+	fprintf(stderr, "       %s increase\n", name);
+	fprintf(stderr, "       %s set percentage\n", name);
 	exit(1);
 }
 
@@ -117,7 +135,7 @@
 
 #define SYS_PATH "/sys/class/backlight"
 
-static int
+static unsigned int
 read_int(int dfd, const char *file)
 {
 	char buf[32] = {0};
@@ -134,7 +152,7 @@
 }
 
 static int
-write_int(int dfd, const char *file, int value)
+write_int(int dfd, const char *file, unsigned int value)
 {
 	char buf[32] = {0};
 	int length;
@@ -191,34 +209,66 @@
 	closedir(dirp);
 	close(dfd);
 
+	if (adaptorfd < 0)
+		die("could not find card adaptor\n");
+
 	return adaptorfd;
 }
 
-static int
-set(int type)
+static unsigned int
+downscale(int dfd, unsigned int value)
 {
-	int current, max;
-	int dfd;
+	return value * 100 / read_int(dfd, "max_brightness");
+}
+
+static unsigned int
+upscale(int dfd, unsigned int value)
+{
+	return value * read_int(dfd, "max_brightness") / 100;
+}
 
-	if ((dfd = find_adaptor()) < 0)
-		die("could not find card adaptor\n");
+static void
+set(unsigned int percent)
+{
+	int dfd = find_adaptor();
+	unsigned int value = upscale(dfd, percent);
 
-	current = read_int(dfd, "actual_brightness");
-	max = read_int(dfd, "max_brightness");
+	write_int(dfd, "brightness", value);
+}
+
+static void
+increase(void)
+{
+	unsigned int value = get();
 
-	if (type == Up) {
-		current += max / 100;
+	if (value + 2 > 100)
+		value = 100;
+	else
+		value += 2;
+
+	set(value);
+}
+
+static void
+decrease(void)
+{
+	unsigned int value = get();
 
-		if (current > max)
-			return max;
-	} else {
-		current -= max / 100;
+	if ((int)value - 2 < 0)
+		value = 0;
+	else
+		value -= 2;
+
+	set(value);
+}
 
-		if (current < 0)
-			return 0;
-	}
+static unsigned int
+get(void)
+{
+	int dfd = find_adaptor();
+	int value = read_int(dfd, "brightness");
 
-	return write_int(dfd, "brightness", current);
+	return downscale(dfd, value);
 }
 
 #else
@@ -230,8 +280,6 @@
 	(void)type;
 
 	die("backlight is not supported on this system");
-
-	return -1;
 }
 
 #endif
@@ -241,20 +289,21 @@
 int
 main(int argc, char *argv[])
 {
-	int st, type = None;
-
-	if (argc < 2)
-		usage("backlight");
+	-- argc;
+	++ argv;
 
-	if (strcmp(argv[1], "up") == 0)
-		type = Up;
-	else if (strcmp(argv[1], "down") == 0)
-		type = Down;
-	else
+	if (argc < 1)
 		usage(argv[0]);
 
-	if ((st = set(type)) < 0)
-		return 1;
+	if (strcmp(argv[0], "get") == 0)
+		printf("%d\n", get());
+	else if (strcmp(argv[0], "set") == 0) {
+		if (argc != 2)
+			usage(argv[0]);
 
-	printf("switching to %d\n", st);
+		set(atoi(argv[1]));
+	} else if (strcmp(argv[0], "increase") == 0)
+		increase();
+	else if (strcmp(argv[0], "decrease") == 0)
+		decrease();
 }