Provide transition duration for ui side dimming

The enabler for ui side dimming was already implemented earlier, but
turns out it does provide enough information and had some glitches.

In addition to fade-to-black -level the signal contains also the
transition duration that ui side should use for the level change.
This allows using mce brightness fading timing configuration also
for the transitions happening at lipstick side.

To avoid unwanted brightness jumps during display power up, the ui
side fading level is retained over display off periods. Whether it
is still needed after display power up is evaluated when als data
becomes available.

Fading by 70% made the display too dark - adjusted to 50%.

[mce] Provide transition duration for ui side dimming. Contributes to JB#18412

modules/display.c

@@ -87,7 +87,7 @@
  * backlight dimming alone is not enough to make dimmed display state
  * visible to the user.
  */
-#define MCE_FADER_OPACITY_PERCENT 70
+#define MCE_FADER_OPACITY_PERCENT 50
 
 /* ========================================================================= *
  * TYPEDEFS
@@ -1233,8 +1233,6 @@ static void mdy_datapipe_display_state_cb(gconstpointer data)
     mce_log(LL_DEVEL, "current display state = %s",
             mdy_display_state_name(display_state));
 
-    mdy_ui_dimming_rethink();
-
 EXIT:
     return;
 }
@@ -2787,44 +2785,89 @@ static void mdy_ui_dimming_rethink(void)
      * evaluation on mce startup forces signal to be sent */
     static dbus_int32_t dimming_prev = -1;
 
-    /* By default the ui side dimming should not occur */
+    /* This gets a bit hairy because we do not want to restart the
+     * ui side fade animation once it has started and heading to
+     * the correct level -> on display power up we want to make
+     * only one guess when/if the fading target changes and how
+     * fast the change should happen.
+     *
+     * The triggers for calling this function are:
+     * 1) display state transition starts
+     * 2) als tuning changes mdy_ui_dimming_is_needed
+     *
+     * When (1) happens, both display_state and display_state_next
+     * hold stable states.
+     *
+     * If (2) happens during display power up/down, the
+     * display_state variable can hold transitient
+     * MCE_DISPLAY_POWER_UP/DOWN states.
+     */
+
+    /* Assume that ui side dimming should not occur */
     dbus_int32_t dimming_curr = 0;
 
-    if( mdy_ui_dimming_is_needed ) {
-        /* Backlight brightness tuning would not produce visible difference
-         * between ON and DIM display state */
-
-        if( display_state == MCE_DISPLAY_DIM ||
-            ( display_state_next == MCE_DISPLAY_DIM &&
-              display_state == MCE_DISPLAY_ON ) ) {
-            /* We are in dimmed display state, or making transtion to
-             * dimmed state from on state -> signal ui side that it
-             * should do MCE_FADER_OPACITY_PERCENT fade to black.
-             *
-             * Note that we explicitly do not want to
-             * a) start ui side fading while powering display up
-             *    to dimmed state (might cause unwanted ui side cpu
-             *    load and should not occur in normal use anyway)
-             * b) stop ui side fading while powering display down
-             *    from dimmed state (might cause flickering if ui
-             *    side would start to make display brighter while
-             *    mce is ramping down the backlight brightness)
-             */
+    if( display_state == MCE_DISPLAY_POWER_DOWN ||
+        display_state_next == MCE_DISPLAY_OFF   ||
+        display_state_next == MCE_DISPLAY_LPM_OFF ) {
+        /* At or entering powered off state -> keep current state */
+        if( dimming_prev >= 0 )
+            dimming_curr = dimming_prev;
+    }
+    else if( display_state_next == MCE_DISPLAY_DIM ) {
+        /* At or entering dimmed state -> use if needed */
+        if( mdy_ui_dimming_is_needed )
             dimming_curr = MCE_FADER_OPACITY_PERCENT;
-        }
     }
 
+    /* Skip the rest if the target level does not change */
     if( dimming_prev == dimming_curr )
         goto EXIT;
 
     dimming_prev = dimming_curr;
 
-    mce_log(LL_DEVEL, "sending dbus signal: %s %d",
-            MCE_FADER_OPACITY_SIG, dimming_curr);
+    /* Assume the change is due to ALS tuning */
+    dbus_int32_t duration = mdy_brightness_fade_duration_als_ms;
+
+    if( display_state == MCE_DISPLAY_POWER_UP ) {
+        /* Leaving powered off state -> use unblank duration */
+        duration = mdy_brightness_fade_duration_unblank_ms;
+    }
+    else if( display_state == MCE_DISPLAY_POWER_DOWN ) {
+        /* Entering powered off state -> use blank duration */
+        duration = mdy_brightness_fade_duration_blank_ms;
+    }
+    else if( display_state != display_state_next ) {
+        /* Ongoing display state transition that does not need
+         * or has not yet entered transient state */
+        if( display_state == MCE_DISPLAY_OFF ||
+            display_state == MCE_DISPLAY_LPM_OFF ) {
+            /* Leaving powered off state -> use unblank duration */
+            duration = mdy_brightness_fade_duration_unblank_ms;
+        }
+        else if( display_state_next == MCE_DISPLAY_OFF ||
+                 display_state_next == MCE_DISPLAY_LPM_OFF ) {
+            /* Entering powered off state -> use blank duration */
+            duration = mdy_brightness_fade_duration_blank_ms;
+        }
+        else if( display_state_next == MCE_DISPLAY_DIM ) {
+            /* Entering dimmed state -> use dimming duration */
+            duration = mdy_brightness_fade_duration_dim_ms;
+        }
+        else {
+            /* Use default state transition duration */
+            duration = mdy_brightness_fade_duration_def_ms;
+        }
+    }
+
+    mce_log(LL_DEVEL, "sending dbus signal: %s %d %d",
+            MCE_FADER_OPACITY_SIG, dimming_curr, duration);
+
     dbus_send(0, MCE_SIGNAL_PATH, MCE_SIGNAL_IF,
               MCE_FADER_OPACITY_SIG, 0,
               DBUS_TYPE_INT32, &dimming_curr,
+              DBUS_TYPE_INT32, &duration,
               DBUS_TYPE_INVALID);
+
 EXIT:
     return;
 }
@@ -7494,6 +7537,7 @@ static mce_dbus_handler_t mdy_dbus_handlers[] =
         .type      = DBUS_MESSAGE_TYPE_SIGNAL,
         .args      =
             "    <arg name=\"fader_opacity_percent\" type=\"i\"/>\n"
+            "    <arg name=\"transition_length\" type=\"i\"/>\n"
     },
     /* signals */
     {