Nouveau - Aug 2022 - [PATCH v3 09/31] ACPI: video: Make backlight class device registration a separate step (v2)

On x86/ACPI boards the acpi_video driver will usually initialize before
the kms driver (except i915). This causes /sys/class/backlight/acpi_video0
to show up and then the kms driver registers its own native backlight
device after which the drivers/acpi/video_detect.c code unregisters
the acpi_video0 device (when acpi_video_get_backlight_type()==native).

This means that userspace briefly sees 2 devices and the disappearing of
acpi_video0 after a brief time confuses the systemd backlight level
save/restore code, see e.g.:
https://bbs.archlinux.org/viewtopic.php?id=269920

To fix this make backlight class device registration a separate step
done by a new acpi_video_register_backlight() function. The intend is for
this to be called by the drm/kms driver *after* it is done setting up its
own native backlight device. So that acpi_video_get_backlight_type() knows
if a native backlight will be available or not at acpi_video backlight
registration time, avoiding the add + remove dance.

Note the new acpi_video_register_backlight() function is also called from
a delayed work to ensure that the acpi_video backlight devices does get
registered if necessary even if there is no drm/kms driver or when it is
disabled.

Changes in v2:
- Make register_backlight_delay a module parameter, mainly so that it can
  be disabled by Nvidia binary driver users

Acked-by: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
Signed-off-by: Hans de Goede <hdegoede at redhat.com>
---
 drivers/acpi/acpi_video.c | 50 ++++++++++++++++++++++++++++++++++++---
 include/acpi/video.h      |  2 ++
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
index 8545bf94866f..09dd86f86cf3 100644
--- a/drivers/acpi/acpi_video.c
+++ b/drivers/acpi/acpi_video.c
@@ -73,6 +73,16 @@ module_param(device_id_scheme, bool, 0444);
 static int only_lcd = -1;
 module_param(only_lcd, int, 0444);
 
+/*
+ * Display probing is known to take up to 5 seconds, so delay the fallback
+ * backlight registration by 5 seconds + 3 seconds for some extra margin.
+ */
+static int register_backlight_delay = 8;
+module_param(register_backlight_delay, int, 0444);
+MODULE_PARM_DESC(register_backlight_delay,
+	"Delay in seconds before doing fallback (non GPU driver triggered) "
+	"backlight registration, set to 0 to disable.");
+
 static bool may_report_brightness_keys;
 static int register_count;
 static DEFINE_MUTEX(register_count_mutex);
@@ -81,6 +91,9 @@ static LIST_HEAD(video_bus_head);
 static int acpi_video_bus_add(struct acpi_device *device);
 static int acpi_video_bus_remove(struct acpi_device *device);
 static void acpi_video_bus_notify(struct acpi_device *device, u32 event);
+static void acpi_video_bus_register_backlight_work(struct work_struct
*ignored);
+static DECLARE_DELAYED_WORK(video_bus_register_backlight_work,
+			    acpi_video_bus_register_backlight_work);
 void acpi_video_detect_exit(void);
 
 /*
@@ -1859,8 +1872,6 @@ static int acpi_video_bus_register_backlight(struct
acpi_video_bus *video)
 	if (video->backlight_registered)
 		return 0;
 
-	acpi_video_run_bcl_for_osi(video);
-
 	if (acpi_video_get_backlight_type() != acpi_backlight_video)
 		return 0;
 
@@ -2086,7 +2097,11 @@ static int acpi_video_bus_add(struct acpi_device *device)
 	list_add_tail(&video->entry, &video_bus_head);
 	mutex_unlock(&video_list_lock);
 
-	acpi_video_bus_register_backlight(video);
+	/*
+	 * The userspace visible backlight_device gets registered separately
+	 * from acpi_video_register_backlight().
+	 */
+	acpi_video_run_bcl_for_osi(video);
 	acpi_video_bus_add_notify_handler(video);
 
 	return 0;
@@ -2125,6 +2140,11 @@ static int acpi_video_bus_remove(struct acpi_device
*device)
 	return 0;
 }
 
+static void acpi_video_bus_register_backlight_work(struct work_struct *ignored)
+{
+	acpi_video_register_backlight();
+}
+
 static int __init is_i740(struct pci_dev *dev)
 {
 	if (dev->device == 0x00D1)
@@ -2235,6 +2255,18 @@ int acpi_video_register(void)
 	 */
 	register_count = 1;
 
+	/*
+	 * acpi_video_bus_add() skips registering the userspace visible
+	 * backlight_device. The intend is for this to be registered by the
+	 * drm/kms driver calling acpi_video_register_backlight() *after* it is
+	 * done setting up its own native backlight device. The delayed work
+	 * ensures that acpi_video_register_backlight() always gets called
+	 * eventually, in case there is no drm/kms driver or it is disabled.
+	 */
+	if (register_backlight_delay)
+		schedule_delayed_work(&video_bus_register_backlight_work,
+				      register_backlight_delay * HZ);
+
 leave:
 	mutex_unlock(&register_count_mutex);
 	return ret;
@@ -2245,6 +2277,7 @@ void acpi_video_unregister(void)
 {
 	mutex_lock(&register_count_mutex);
 	if (register_count) {
+		cancel_delayed_work_sync(&video_bus_register_backlight_work);
 		acpi_bus_unregister_driver(&acpi_video_bus);
 		register_count = 0;
 		may_report_brightness_keys = false;
@@ -2253,6 +2286,17 @@ void acpi_video_unregister(void)
 }
 EXPORT_SYMBOL(acpi_video_unregister);
 
+void acpi_video_register_backlight(void)
+{
+	struct acpi_video_bus *video;
+
+	mutex_lock(&video_list_lock);
+	list_for_each_entry(video, &video_bus_head, entry)
+		acpi_video_bus_register_backlight(video);
+	mutex_unlock(&video_list_lock);
+}
+EXPORT_SYMBOL(acpi_video_register_backlight);
+
 void acpi_video_unregister_backlight(void)
 {
 	struct acpi_video_bus *video;
diff --git a/include/acpi/video.h b/include/acpi/video.h
index 4705e339c252..0625806d3bbd 100644
--- a/include/acpi/video.h
+++ b/include/acpi/video.h
@@ -53,6 +53,7 @@ enum acpi_backlight_type {
 #if IS_ENABLED(CONFIG_ACPI_VIDEO)
 extern int acpi_video_register(void);
 extern void acpi_video_unregister(void);
+extern void acpi_video_register_backlight(void);
 extern int acpi_video_get_edid(struct acpi_device *device, int type,
 			       int device_id, void **edid);
 extern enum acpi_backlight_type acpi_video_get_backlight_type(void);
@@ -69,6 +70,7 @@ extern int acpi_video_get_levels(struct acpi_device *device,
 #else
 static inline int acpi_video_register(void) { return -ENODEV; }
 static inline void acpi_video_unregister(void) { return; }
+static inline void acpi_video_register_backlight(void) { return; }
 static inline int acpi_video_get_edid(struct acpi_device *device, int type,
 				      int device_id, void **edid)
 {
-- 
2.37.2

On 8/18/22 1:42 PM, Hans de Goede wrote:
> On x86/ACPI boards the acpi_video driver will usually initialize before
> the kms driver (except i915). This causes /sys/class/backlight/acpi_video0
> to show up and then the kms driver registers its own native backlight
> device after which the drivers/acpi/video_detect.c code unregisters
> the acpi_video0 device (when acpi_video_get_backlight_type()==native).
>
> This means that userspace briefly sees 2 devices and the disappearing of
> acpi_video0 after a brief time confuses the systemd backlight level
> save/restore code, see e.g.:
> https://bbs.archlinux.org/viewtopic.php?id=269920
>
> To fix this make backlight class device registration a separate step
> done by a new acpi_video_register_backlight() function. The intend is for
> this to be called by the drm/kms driver *after* it is done setting up its
> own native backlight device. So that acpi_video_get_backlight_type() knows
> if a native backlight will be available or not at acpi_video backlight
> registration time, avoiding the add + remove dance.
>
> Note the new acpi_video_register_backlight() function is also called from
> a delayed work to ensure that the acpi_video backlight devices does get
> registered if necessary even if there is no drm/kms driver or when it is
> disabled.
>
> Changes in v2:
> - Make register_backlight_delay a module parameter, mainly so that it can
>    be disabled by Nvidia binary driver users
>
> Acked-by: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
>   drivers/acpi/acpi_video.c | 50 ++++++++++++++++++++++++++++++++++++---
>   include/acpi/video.h      |  2 ++
>   2 files changed, 49 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
> index 8545bf94866f..09dd86f86cf3 100644
> --- a/drivers/acpi/acpi_video.c
> +++ b/drivers/acpi/acpi_video.c
> @@ -73,6 +73,16 @@ module_param(device_id_scheme, bool, 0444);
>   static int only_lcd = -1;
>   module_param(only_lcd, int, 0444);
>   
> +/*
> + * Display probing is known to take up to 5 seconds, so delay the fallback
> + * backlight registration by 5 seconds + 3 seconds for some extra margin.
> + */
> +static int register_backlight_delay = 8;
> +module_param(register_backlight_delay, int, 0444);

Would it make sense to make this parameter writable from userspace, e.g. 
so that it can be set by a udev rule rather than relying on a riskier 
kernel command line edit? Then again, that probably makes things more 
complicated, since you'd have to check the parameter again when the 
worker fires, and changing the parameter to a non-zero value from either 
zero or a different non-zero value would be too weird. And making a 
separate writable parameter to allow userspace to turn the worker into a 
noop despite it being enabled when the kernel was initially loaded seems 
wrong, too.

> +MODULE_PARM_DESC(register_backlight_delay,
> +	"Delay in seconds before doing fallback (non GPU driver triggered)
"
> +	"backlight registration, set to 0 to disable.");
> +
>   static bool may_report_brightness_keys;
>   static int register_count;
>   static DEFINE_MUTEX(register_count_mutex);
> @@ -81,6 +91,9 @@ static LIST_HEAD(video_bus_head);
>   static int acpi_video_bus_add(struct acpi_device *device);
>   static int acpi_video_bus_remove(struct acpi_device *device);
>   static void acpi_video_bus_notify(struct acpi_device *device, u32 event);
> +static void acpi_video_bus_register_backlight_work(struct work_struct
*ignored);
> +static DECLARE_DELAYED_WORK(video_bus_register_backlight_work,
> +			    acpi_video_bus_register_backlight_work);
>   void acpi_video_detect_exit(void);
>   
>   /*
> @@ -1859,8 +1872,6 @@ static int acpi_video_bus_register_backlight(struct
acpi_video_bus *video)
>   	if (video->backlight_registered)
>   		return 0;
>   
> -	acpi_video_run_bcl_for_osi(video);
> -
>   	if (acpi_video_get_backlight_type() != acpi_backlight_video)
>   		return 0;
>   
> @@ -2086,7 +2097,11 @@ static int acpi_video_bus_add(struct acpi_device
*device)
>   	list_add_tail(&video->entry, &video_bus_head);
>   	mutex_unlock(&video_list_lock);
>   
> -	acpi_video_bus_register_backlight(video);
> +	/*
> +	 * The userspace visible backlight_device gets registered separately
> +	 * from acpi_video_register_backlight().
> +	 */
> +	acpi_video_run_bcl_for_osi(video);
>   	acpi_video_bus_add_notify_handler(video);
>   
>   	return 0;
> @@ -2125,6 +2140,11 @@ static int acpi_video_bus_remove(struct acpi_device
*device)
>   	return 0;
>   }
>   
> +static void acpi_video_bus_register_backlight_work(struct work_struct
*ignored)
> +{
> +	acpi_video_register_backlight();
> +}
> +
>   static int __init is_i740(struct pci_dev *dev)
>   {
>   	if (dev->device == 0x00D1)
> @@ -2235,6 +2255,18 @@ int acpi_video_register(void)
>   	 */
>   	register_count = 1;
>   
> +	/*
> +	 * acpi_video_bus_add() skips registering the userspace visible
> +	 * backlight_device. The intend is for this to be registered by the
> +	 * drm/kms driver calling acpi_video_register_backlight() *after* it is
> +	 * done setting up its own native backlight device. The delayed work
> +	 * ensures that acpi_video_register_backlight() always gets called
> +	 * eventually, in case there is no drm/kms driver or it is disabled.
> +	 */
> +	if (register_backlight_delay)
> +		schedule_delayed_work(&video_bus_register_backlight_work,
> +				      register_backlight_delay * HZ);
> +
>   leave:
>   	mutex_unlock(&register_count_mutex);
>   	return ret;
> @@ -2245,6 +2277,7 @@ void acpi_video_unregister(void)
>   {
>   	mutex_lock(&register_count_mutex);
>   	if (register_count) {
> +		cancel_delayed_work_sync(&video_bus_register_backlight_work);
>   		acpi_bus_unregister_driver(&acpi_video_bus);
>   		register_count = 0;
>   		may_report_brightness_keys = false;
> @@ -2253,6 +2286,17 @@ void acpi_video_unregister(void)
>   }
>   EXPORT_SYMBOL(acpi_video_unregister);
>   
> +void acpi_video_register_backlight(void)
> +{
> +	struct acpi_video_bus *video;
> +
> +	mutex_lock(&video_list_lock);
> +	list_for_each_entry(video, &video_bus_head, entry)
> +		acpi_video_bus_register_backlight(video);
> +	mutex_unlock(&video_list_lock);
> +}
> +EXPORT_SYMBOL(acpi_video_register_backlight);
> +
>   void acpi_video_unregister_backlight(void)
>   {
>   	struct acpi_video_bus *video;
> diff --git a/include/acpi/video.h b/include/acpi/video.h
> index 4705e339c252..0625806d3bbd 100644
> --- a/include/acpi/video.h
> +++ b/include/acpi/video.h
> @@ -53,6 +53,7 @@ enum acpi_backlight_type {
>   #if IS_ENABLED(CONFIG_ACPI_VIDEO)
>   extern int acpi_video_register(void);
>   extern void acpi_video_unregister(void);
> +extern void acpi_video_register_backlight(void);
>   extern int acpi_video_get_edid(struct acpi_device *device, int type,
>   			       int device_id, void **edid);
>   extern enum acpi_backlight_type acpi_video_get_backlight_type(void);
> @@ -69,6 +70,7 @@ extern int acpi_video_get_levels(struct acpi_device
*device,
>   #else
>   static inline int acpi_video_register(void) { return -ENODEV; }
>   static inline void acpi_video_unregister(void) { return; }
> +static inline void acpi_video_register_backlight(void) { return; }
>   static inline int acpi_video_get_edid(struct acpi_device *device, int
type,
>   				      int device_id, void **edid)
>   {