tests: Add backlight sanity test

It currently sanity checks the MetaOutput and MetaMonitor APIs.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3861>

src/backends/meta-monitor.h

@@ -336,6 +336,7 @@ gboolean meta_parse_monitor_mode (const char *string,
                                   float      *out_refresh_rate,
                                   float       fallback_refresh_rate);
 
+META_EXPORT_TEST
 gboolean meta_monitor_get_backlight_info (MetaMonitor *monitor,
                                           int         *backlight_min,
                                           int         *backlight_max);
@@ -343,5 +344,6 @@ gboolean meta_monitor_get_backlight_info (MetaMonitor *monitor,
 void meta_monitor_set_backlight (MetaMonitor *monitor,
                                  int          value);
 
+META_EXPORT_TEST
 gboolean meta_monitor_get_backlight (MetaMonitor *monitor,
                                      int         *value);

src/backends/meta-output.h

@@ -258,9 +258,11 @@ META_EXPORT_TEST
 gboolean meta_output_get_max_bpc (MetaOutput   *output,
                                   unsigned int *max_bpc);
 
+META_EXPORT_TEST
 void meta_output_set_backlight (MetaOutput *output,
                                 int         backlight);
 
+META_EXPORT_TEST
 int meta_output_get_backlight (MetaOutput *output);
 
 MetaPrivacyScreenState meta_output_get_privacy_screen_state (MetaOutput *output);

src/tests/meson.build

@@ -271,6 +271,11 @@ test_cases += [
     'suite': 'unit',
     'sources': [ 'monitor-util-tests.c', ],
   },
+  {
+    'name': 'monitor-backlight',
+    'suite': 'backend',
+    'sources': [ 'monitor-backlight-tests.c', ]
+  },
   {
     'name': 'stage-views',
     'suite': 'compositor',

src/tests/meta-monitor-test-utils.c

@@ -771,6 +771,8 @@ meta_create_monitor_test_setup (MetaBackend          *backend,
           output_info->suggested_x = -1;
           output_info->suggested_y = -1;
         }
+      output_info->backlight_min = setup->outputs[i].backlight_min;
+      output_info->backlight_max = setup->outputs[i].backlight_max;
       output_info->width_mm = setup->outputs[i].width_mm;
       output_info->height_mm = setup->outputs[i].height_mm;
       output_info->subpixel_order = META_SUBPIXEL_ORDER_UNKNOWN;

src/tests/meta-monitor-test-utils.h

@@ -113,6 +113,8 @@ typedef struct _MonitorTestCaseOutput
   gboolean hotplug_mode;
   int suggested_x;
   int suggested_y;
+  int backlight_min;
+  int backlight_max;
   gboolean has_edid_info;
   MetaEdidInfo edid_info;
 } MonitorTestCaseOutput;

src/tests/meta-output-test.c

@@ -21,9 +21,50 @@
 
 G_DEFINE_TYPE (MetaOutputTest, meta_output_test, META_TYPE_OUTPUT_NATIVE)
 
+static void
+on_backlight_changed (MetaOutput *output)
+{
+  const MetaOutputInfo *info = meta_output_get_info (output);
+  int value = meta_output_get_backlight (output);
+
+  g_assert_cmpint (info->backlight_min, <=, value);
+  g_assert_cmpint (info->backlight_max, >=, value);
+}
+
+static void
+meta_output_test_constructed (GObject *object)
+{
+  MetaOutput *output = META_OUTPUT (object);
+  const MetaOutputInfo *info = meta_output_get_info (output);
+
+  if (info->backlight_min != info->backlight_max)
+    {
+      int backlight_range = info->backlight_max - info->backlight_min;
+
+      meta_output_set_backlight (output,
+                                 info->backlight_min + backlight_range / 2);
+      g_signal_connect (output, "backlight-changed",
+                        G_CALLBACK (on_backlight_changed), NULL);
+    }
+
+  G_OBJECT_CLASS (meta_output_test_parent_class)->constructed (object);
+}
+
+static GBytes *
+meta_output_test_read_edid (MetaOutputNative *output_native)
+{
+  return NULL;
+}
+
 static void
 meta_output_test_class_init (MetaOutputTestClass *klass)
 {
+  GObjectClass *object_class = G_OBJECT_CLASS (klass);
+  MetaOutputNativeClass *output_native_class = META_OUTPUT_NATIVE_CLASS (klass);
+
+  object_class->constructed = meta_output_test_constructed;
+
+  output_native_class->read_edid = meta_output_test_read_edid;
 }
 
 static void

src/tests/monitor-backlight-tests.c

@@ -0,0 +1,149 @@
+/*
+ * Copyright (C) 2025 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "config.h"
+
+#include <stdio.h>
+
+#include "tests/meta-test/meta-context-test.h"
+#include "tests/meta-monitor-test-utils.h"
+
+static MonitorTestCaseSetup initial_test_case_setup = {
+  .modes = {
+    {
+      .width = 1024,
+      .height = 768,
+      .refresh_rate = 60.0
+    }
+  },
+  .n_modes = 1,
+  .outputs = {
+     {
+      .crtc = 0,
+      .modes = { 0 },
+      .n_modes = 1,
+      .preferred_mode = 0,
+      .possible_crtcs = { 0 },
+      .n_possible_crtcs = 1,
+      .width_mm = 222,
+      .height_mm = 125,
+      .is_laptop_panel = TRUE,
+      .backlight_min = 0,
+      .backlight_max = 300,
+    },
+    {
+      .crtc = 1,
+      .modes = { 0 },
+      .n_modes = 1,
+      .preferred_mode = 0,
+      .possible_crtcs = { 1 },
+      .n_possible_crtcs = 1,
+      .width_mm = 220,
+      .height_mm = 124
+    }
+  },
+  .n_outputs = 2,
+  .crtcs = {
+    {
+      .current_mode = 0
+    },
+    {
+      .current_mode = 0
+    }
+  },
+  .n_crtcs = 2
+};
+
+static MetaContext *test_context;
+
+static MetaMonitorTestSetup *
+create_test_setup (MetaBackend *backend)
+{
+  return meta_create_monitor_test_setup (backend,
+                                         &initial_test_case_setup,
+                                         MONITOR_TEST_FLAG_NO_STORED);
+}
+
+static void
+meta_test_backlight_sanity (void)
+{
+  MetaBackend *backend = meta_context_get_backend (test_context);
+  MetaMonitorManager *monitor_manager =
+    meta_backend_get_monitor_manager (backend);
+  GList *monitors;
+  MetaMonitor *first_monitor;
+  MetaMonitor *second_monitor;
+  MetaOutput *output;
+  const MetaOutputInfo *output_info;
+  int backlight_min;
+  int backlight_max;
+  int backlight;
+
+  monitors = meta_monitor_manager_get_monitors (monitor_manager);
+  g_assert_cmpuint (g_list_length (monitors), ==, 2);
+
+  first_monitor = g_list_nth_data (monitors, 0);
+  second_monitor = g_list_nth_data (monitors, 1);
+
+  g_assert_true (meta_monitor_get_backlight_info (first_monitor,
+                                                  &backlight_min,
+                                                  &backlight_max));
+  g_assert_cmpint (backlight_min, ==, 0);
+  g_assert_cmpint (backlight_max, ==, 300);
+  g_assert_true (meta_monitor_get_backlight (first_monitor, &backlight));
+  g_assert_cmpint (backlight, >=, 0);
+  g_assert_cmpuint (g_list_length (meta_monitor_get_outputs (first_monitor)),
+                    ==,
+                    1);
+  output = meta_monitor_get_main_output (first_monitor);
+  output_info = meta_output_get_info (output);
+  g_assert_cmpint (meta_output_get_backlight (output), >=, 0);
+  g_assert_cmpint (output_info->backlight_min, ==, 0);
+  g_assert_cmpint (output_info->backlight_max, ==, 300);
+
+  g_assert_false (meta_monitor_get_backlight_info (second_monitor,
+                                                   NULL,
+                                                   NULL));
+  g_assert_false (meta_monitor_get_backlight (second_monitor, NULL));
+  g_assert_cmpuint (g_list_length (meta_monitor_get_outputs (second_monitor)),
+                    ==,
+                    1);
+  output = meta_monitor_get_main_output (second_monitor);
+  output_info = meta_output_get_info (output);
+  g_assert_cmpint (meta_output_get_backlight (output), ==, -1);
+  g_assert_cmpint (output_info->backlight_min, ==, 0);
+  g_assert_cmpint (output_info->backlight_max, ==, 0);
+}
+
+int
+main (int   argc,
+      char *argv[])
+{
+  g_autoptr (MetaContext) context = NULL;
+
+  context = meta_create_test_context (META_CONTEXT_TEST_TYPE_TEST,
+                                      META_CONTEXT_TEST_FLAG_TEST_CLIENT);
+  g_assert (meta_context_configure (context, &argc, &argv, NULL));
+
+  test_context = context;
+
+  meta_init_monitor_test_setup (create_test_setup);
+  g_test_add_func ("/backends/backlight/sanity", meta_test_backlight_sanity);
+
+  return meta_context_test_run_tests (META_CONTEXT_TEST (context),
+                                      META_TEST_RUN_FLAG_NONE);
+}