ddcci_backlight: Add convenience symlink to ancestor device in sysfs

Add a symlink `ddcci_backlight` to the i2c bus' parent device.
For many graphics drivers that's the DRM device node in sysfs.
If multiple backlights have the same ancestor device, the first
one wins.

Can be disabled using the module parameter `convenience_symlink`.

(Feature request torvalds#13)

ddcci-backlight/ddcci-backlight.c

@@ -15,6 +15,7 @@
 #include <linux/backlight.h>
 #include <linux/module.h>
 #include <linux/fb.h>
+#include <linux/sysfs.h>
 
 #include <linux/ddcci.h>
 
@@ -28,6 +29,8 @@
 #define DDCCI_MONITOR_BACKLIGHT	0x13
 #define DDCCI_MONITOR_BL_WHITE		0x6B
 
+static bool convenience_symlink = true;
+
 struct ddcci_monitor_drv_data {
 	struct ddcci_device *device;
 	struct backlight_device *bl_dev;
@@ -207,6 +210,68 @@ static bool ddcci_monitor_find_vcp(unsigned char vcp, const char *s)
 	return false;
 }
 
+static int ddcci_backlight_create_symlink(struct ddcci_device *ddcci_dev)
+{
+	int i, result;
+	struct device *dev = &ddcci_dev->dev;
+	struct kernfs_node *dirent;
+	for (i = 0; i < 3; ++i) {
+		dev = dev->parent;
+		if (!dev) {
+			dev_dbg(&ddcci_dev->dev, "failed to create convenience symlink: ancestor device not found\n");
+			return -ENOENT;
+		}
+	}
+	dirent = sysfs_get_dirent(dev->kobj.sd, "ddcci_backlight");
+	if (dirent) {
+		sysfs_put(dirent);
+		dev_dbg(&ddcci_dev->dev, "failed to create convenience symlink: %s/ddcci_backlight already exists\n", dev_name(dev));
+		return -EEXIST;
+	}
+
+	result = sysfs_create_link(&dev->kobj, &ddcci_dev->dev.kobj, "ddcci_backlight");
+	if (result == 0) {
+		dev_dbg(&ddcci_dev->dev, "created symlink %s/ddcci_backlight\n", dev_name(dev));
+	} else {
+		dev_info(&ddcci_dev->dev, "failed to create convenience symlink: %d\n", result);
+	}
+	return result;
+}
+
+static int ddcci_backlight_remove_symlink(struct ddcci_device *ddcci_dev)
+{
+	int i;
+	struct device *dev = &ddcci_dev->dev;
+	struct kernfs_node *dirent;
+	for (i = 0; i < 3; ++i) {
+		dev = dev->parent;
+		if (!dev)
+			return -ENOENT;
+	}
+	dirent = sysfs_get_dirent(dev->kobj.sd, "ddcci_backlight");
+	if (!dirent) {
+		return -ENOENT;
+	}
+
+	if ((dirent->flags & KERNFS_LINK) == 0) {
+		sysfs_put(dirent);
+		dev_dbg(&ddcci_dev->dev, "won't remove %s/ddcci_backlight: not a symlink\n", dev_name(dev));
+		return -EINVAL;
+	}
+
+	if (dirent->symlink.target_kn != ddcci_dev->dev.kobj.sd) {
+		sysfs_put(dirent);
+		dev_dbg(&ddcci_dev->dev, "won't remove %s/ddcci_backlight: we are not the link target\n", dev_name(dev));
+		return -EINVAL;
+	}
+
+	sysfs_put(dirent);
+
+	sysfs_remove_link(&dev->kobj, "ddcci_backlight");
+	dev_dbg(&ddcci_dev->dev, "removed symlink %s/ddcci_backlight\n", dev_name(dev));
+	return 0;
+}
+
 static int ddcci_monitor_probe(struct ddcci_device *dev,
 			       const struct ddcci_device_id *id)
 {
@@ -300,6 +365,10 @@ static int ddcci_monitor_probe(struct ddcci_device *dev,
 		 ddcci_monitor_vcp_name(drv_data->used_vcp),
 		 dev_name(&dev->dev));
 
+	if (convenience_symlink) {
+		ddcci_backlight_create_symlink(dev);
+	}
+
 	goto end;
 err_free:
 	devm_kfree(&dev->dev, drv_data);
@@ -310,6 +379,7 @@ static int ddcci_monitor_probe(struct ddcci_device *dev,
 static int ddcci_monitor_remove(struct ddcci_device *dev)
 {
 	dev_dbg(&dev->dev, "removing device\n");
+	ddcci_backlight_remove_symlink(dev);
 	return 0;
 }
 
@@ -331,6 +401,10 @@ static struct ddcci_driver ddcci_backlight_driver = {
 
 module_ddcci_driver(ddcci_backlight_driver);
 
+/* Module parameter description */
+module_param(convenience_symlink, bool, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(convenience_symlink, "add convenience symlink \"ddcci_backlight\" to ancestor device in sysfs (default true)");
+
 MODULE_AUTHOR("Christoph Grenz");
 MODULE_DESCRIPTION("DDC/CI generic monitor backlight driver");
 MODULE_VERSION("0.3.3");