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 dcc6f159fd94..35ce07b6a5bc 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 c3a89414ddc0..1be358c1448a 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);