[PATCH v8 02/14] backlight: add kernel-internal backlight API

[Mario Limonciello <mario.limonciello@amd.com>]

So far backlights have only been controlled via sysfs. However, sysfs is
not a proper user-space API for runtime modifications, and never was
intended to provide such. The DRM drivers are now prepared to provide
such a backlight link so user-space can control backlight via DRM
connector properties. This allows us to employ the same access-management
we use for mode-setting.

This patch adds a few kernel-internal backlight helpers so we can modify
backlights from within DRM, a brightness-changed notification, and a
per-device takeover count so that legacy sysfs writes can be inhibited
(-EBUSY) while a luminance-aware DRM client is in control.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

V2: Marta Lofstedt <marta.lofstedt@intel.com>
- rebase
- minor edit for checkpatch warning

Signed-off-by: Marta Lofstedt <marta.lofstedt@intel.com>

V3: Mario Limonciello (AMD) <superm1@kernel.org>
 - rebase
 - Use guard(mutex)

V4: Mario Limonciello (AMD) <superm1@kernel.org>
 - Adjust return type for backlight_set_brightness() to return errors
 - Stop clamping in backlight_set_brightness()
 - Drop backlight_device_lookup()

Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
---
 drivers/video/backlight/backlight.c | 66 +++++++++++++++++++++++++++++
 include/linux/backlight.h           | 45 ++++++++++++++++++++
 2 files changed, 111 insertions(+)

diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
index 4401f6294ccc8..1c700c1e77c3f 100644
--- a/drivers/video/backlight/backlight.c
+++ b/drivers/video/backlight/backlight.c
@@ -126,6 +126,9 @@ static void backlight_generate_event(struct backlight_device *bd,
 	case BACKLIGHT_UPDATE_HOTKEY:
 		envp[0] = "SOURCE=hotkey";
 		break;
+	case BACKLIGHT_UPDATE_DRM:
+		envp[0] = "SOURCE=drm";
+		break;
 	default:
 		envp[0] = "SOURCE=unknown";
 		break;
@@ -150,6 +153,13 @@ static ssize_t bl_power_store(struct device *dev, struct device_attribute *attr,
 	struct backlight_device *bd = to_backlight_device(dev);
 	unsigned long power, old_power;
 
+	/* A luminance-aware DRM client has taken over this backlight; the
+	 * legacy sysfs interface is disabled until the last such client
+	 * goes away.
+	 */
+	if (atomic_read(&bd->drm_takeover) > 0)
+		return -EBUSY;
+
 	rc = kstrtoul(buf, 0, &power);
 	if (rc)
 		return rc;
@@ -214,6 +224,13 @@ static ssize_t brightness_store(struct device *dev,
 	struct backlight_device *bd = to_backlight_device(dev);
 	unsigned long brightness;
 
+	/* A luminance-aware DRM client has taken over this backlight; the
+	 * legacy sysfs interface is disabled until the last such client
+	 * goes away.
+	 */
+	if (atomic_read(&bd->drm_takeover) > 0)
+		return -EBUSY;
+
 	rc = kstrtoul(buf, 0, &brightness);
 	if (rc)
 		return rc;
@@ -514,6 +531,41 @@ static int devm_backlight_device_match(struct device *dev, void *res,
 	return *r == data;
 }
 
+/**
+ * backlight_set_brightness - set brightness on a backlight device
+ * @bd: backlight device to operate on
+ * @value: brightness value to set on the device
+ * @reason: backlight-change reason to use for notifications
+ *
+ * This is the in-kernel API equivalent of writing into the 'brightness' sysfs
+ * file. It calls into the underlying backlight driver to change the brightness
+ * value.
+ * A uevent notification is sent with the reason set to @reason.
+ * Return: 0 if successfully notified, -EINVAL for invalid values, -ENXIO if the
+ * device has no ops (e.g. it is being unregistered)
+ */
+int backlight_set_brightness(struct backlight_device *bd, unsigned int value,
+			      enum backlight_update_reason reason)
+{
+	int rc = -ENXIO;
+
+	scoped_guard(mutex, &bd->ops_lock) {
+		if (bd->ops) {
+			if (value > bd->props.max_brightness)
+				return -EINVAL;
+
+			dev_dbg(&bd->dev, "set brightness to %u\n", value);
+			bd->props.brightness = value;
+			rc = backlight_update_status(bd);
+		}
+	}
+	if (rc == 0)
+		backlight_generate_event(bd, reason);
+
+	return rc;
+}
+EXPORT_SYMBOL_GPL(backlight_set_brightness);
+
 /**
  * backlight_register_notifier - get notified of backlight (un)registration
  * @nb: notifier block with the notifier to call on backlight (un)registration
@@ -548,6 +600,20 @@ int backlight_unregister_notifier(struct notifier_block *nb)
 }
 EXPORT_SYMBOL(backlight_unregister_notifier);
 
+/**
+ * backlight_notify_brightness - notify brightness change to listeners
+ * @bd: backlight device that changed
+ *
+ * Notify registered listeners that the backlight brightness has changed.
+ * This is called automatically after successful brightness updates.
+ */
+void backlight_notify_brightness(struct backlight_device *bd)
+{
+	blocking_notifier_call_chain(&backlight_notifier,
+				     BACKLIGHT_BRIGHTNESS_CHANGED, bd);
+}
+EXPORT_SYMBOL(backlight_notify_brightness);
+
 /**
  * devm_backlight_device_register - register a new backlight device
  * @dev: the device to register
diff --git a/include/linux/backlight.h b/include/linux/backlight.h
index 015183d129f96..fac1acbc698a1 100644
--- a/include/linux/backlight.h
+++ b/include/linux/backlight.h
@@ -31,6 +31,12 @@ enum backlight_update_reason {
 	 * @BACKLIGHT_UPDATE_SYSFS: The backlight was updated using sysfs.
 	 */
 	BACKLIGHT_UPDATE_SYSFS,
+
+	/**
+	 * @BACKLIGHT_UPDATE_DRM: The backlight was updated from DRM, i.e. through
+	 * a connector LUMINANCE property rather than the legacy sysfs interface.
+	 */
+	BACKLIGHT_UPDATE_DRM,
 };
 
 /**
@@ -82,6 +88,11 @@ enum backlight_notification {
 	 * @BACKLIGHT_UNREGISTERED: The backlight revice is unregistered.
 	 */
 	BACKLIGHT_UNREGISTERED,
+
+	/**
+	 * @BACKLIGHT_BRIGHTNESS_CHANGED: The backlight brightness has changed.
+	 */
+	BACKLIGHT_BRIGHTNESS_CHANGED,
 };
 
 /** enum backlight_scale - the type of scale used for brightness values
@@ -310,8 +321,23 @@ struct backlight_device {
 	 * @use_count: The number of unblanked displays.
 	 */
 	int use_count;
+
+	/**
+	 * @drm_takeover: Number of luminance-aware DRM clients that have
+	 * taken over brightness control of this device. When non-zero,
+	 * writes to the legacy sysfs ``brightness`` attribute return
+	 * ``-EBUSY``. Managed by the DRM backlight helpers.
+	 */
+	atomic_t drm_takeover;
 };
 
+/* Forward declaration for backlight_update_status */
+#if IS_REACHABLE(CONFIG_BACKLIGHT_CLASS_DEVICE)
+void backlight_notify_brightness(struct backlight_device *bd);
+#else
+static inline void backlight_notify_brightness(struct backlight_device *bd) {}
+#endif
+
 /**
  * backlight_update_status - force an update of the backlight device status
  * @bd: the backlight device
@@ -325,6 +351,10 @@ static inline int backlight_update_status(struct backlight_device *bd)
 		ret = bd->ops->update_status(bd);
 	mutex_unlock(&bd->update_lock);
 
+	/* Notify DRM and other listeners that brightness changed */
+	if (ret == 0)
+		backlight_notify_brightness(bd);
+
 	return ret;
 }
 
@@ -431,6 +461,21 @@ static inline void backlight_notify_blank_all(struct device *display_dev,
 { }
 #endif
 
+int backlight_set_brightness(struct backlight_device *bd, unsigned int value,
+			      enum backlight_update_reason reason);
+
+static inline void backlight_device_ref(struct backlight_device *bd)
+{
+	if (bd)
+		get_device(&bd->dev);
+}
+
+static inline void backlight_device_unref(struct backlight_device *bd)
+{
+	if (bd)
+		put_device(&bd->dev);
+}
+
 #define to_backlight_device(obj) container_of(obj, struct backlight_device, dev)
 
 /**
-- 
2.43.0