On Mon, 07 Sep 2026, Mario Limonciello <[email protected]> wrote:
> 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 <[email protected]>
>
> V2: Marta Lofstedt <[email protected]>
> - rebase
> - minor edit for checkpatch warning
>
> Signed-off-by: Marta Lofstedt <[email protected]>
>
> V3: Mario Limonciello (AMD) <[email protected]>
> - rebase
> - Use guard(mutex)
>
> V4: Mario Limonciello (AMD) <[email protected]>
> - 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) <[email protected]>
> ---
> 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)
I think IS_REACHABLE() is almost always a mistake. It's a hack to avoid
link errors when something in a module needs something that is
built-in. It's a combo that should be handled at Kconfig level, not at
build level.
BR,
Jani.
> +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)
>
> /**
--
Jani Nikula, Intel
