backlight: Fix external uses of backlight internal semaphore

backlight_device->sem has a very specific use as documented in the
header file. The external users of this are using it for a different
reason, to serialise access to the update_status() method.

backlight users were supposed to implement their own internal
serialisation of update_status() if needed but everyone is doing
things differently and incorrectly. Therefore add a global mutex to
take care of serialisation for everyone, once and for all.

Locking for get_brightness remains optional since most users don't
need it.

Also update the lcd class in a similar way.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>

2 files changed, 6 insertions, 17 deletions

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index dcc6f15..35ce07b 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -107,12 +107,10 @@ int die(const char *str, struct pt_regs *regs, long err)
 	if (machine_is(powermac) && pmac_backlight) {
 		struct backlight_properties *props;
 
-		down(&pmac_backlight->sem);
 		props = pmac_backlight->props;
 		props->brightness = props->max_brightness;
 		props->power = FB_BLANK_UNBLANK;
-		props->update_status(pmac_backlight);
-		up(&pmac_backlight->sem);
+		backlight_update_status(pmac_backlight);
 	}
 	mutex_unlock(&pmac_backlight_mutex);
 #endif
diff --git a/arch/powerpc/platforms/powermac/backlight.c b/arch/powerpc/platforms/powermac/backlight.c
index c3a8941..1be358c 100644
--- a/arch/powerpc/platforms/powermac/backlight.c
+++ b/arch/powerpc/platforms/powermac/backlight.c
@@ -37,7 +37,9 @@ static int pmac_backlight_set_legacy_queued;
  */
 static atomic_t kernel_backlight_disabled = ATOMIC_INIT(0);
 
-/* Protect the pmac_backlight variable */
+/* Protect the pmac_backlight variable below.
+   You should hold this lock when using the pmac_backlight pointer to
+   prevent its potential removal. */
 DEFINE_MUTEX(pmac_backlight_mutex);
 
 /* Main backlight storage
@@ -49,9 +51,6 @@ DEFINE_MUTEX(pmac_backlight_mutex);
  * internal display, it doesn't matter. Other backlight drivers can be used
  * independently.
  *
- * Lock ordering:
- * pmac_backlight_mutex (global, main backlight)
- *   pmac_backlight->sem (backlight class)
  */
 struct backlight_device *pmac_backlight;
 
@@ -104,7 +103,6 @@ static void pmac_backlight_key_worker(struct work_struct *work)
 		struct backlight_properties *props;
 		int brightness;
 
-		down(&pmac_backlight->sem);
 		props = pmac_backlight->props;
 
 		brightness = props->brightness +
@@ -117,9 +115,7 @@ static void pmac_backlight_key_worker(struct work_struct *work)
 			brightness = props->max_brightness;
 
 		props->brightness = brightness;
-		props->update_status(pmac_backlight);
-
-		up(&pmac_backlight->sem);
+		backlight_update_status(pmac_backlight);
 	}
 	mutex_unlock(&pmac_backlight_mutex);
 }
@@ -145,7 +141,6 @@ static int __pmac_backlight_set_legacy_brightness(int brightness)
 	if (pmac_backlight) {
 		struct backlight_properties *props;
 
-		down(&pmac_backlight->sem);
 		props = pmac_backlight->props;
 		props->brightness = brightness *
 			(props->max_brightness + 1) /
@@ -156,8 +151,7 @@ static int __pmac_backlight_set_legacy_brightness(int brightness)
 		else if (props->brightness < 0)
 			props->brightness = 0;
 
-		props->update_status(pmac_backlight);
-		up(&pmac_backlight->sem);
+		backlight_update_status(pmac_backlight);
 
 		error = 0;
 	}
@@ -196,14 +190,11 @@ int pmac_backlight_get_legacy_brightness()
 	if (pmac_backlight) {
 		struct backlight_properties *props;
 
-		down(&pmac_backlight->sem);
 		props = pmac_backlight->props;
 
 		result = props->brightness *
 			(OLD_BACKLIGHT_MAX + 1) /
 			(props->max_brightness + 1);
-
-		up(&pmac_backlight->sem);
 	}
 	mutex_unlock(&pmac_backlight_mutex);