backend/native: Only disable KMS modifiers for i915

The intel DRM driver is known for not being able to handle multi head
setups when KMS modifiers are enabled, due to the implicitly selected
modifiers, while being more suitable for single head setups, cause
bandwidth issues when a certain number of monitor times resolution and
refresh rate is configured.

We don't yet support automatically finding a combination of modifiers
that work, and have because of this disabled KMS modifiers unless the
driver actually needs it.

Lets flip this configuration the other way around, changing the current
udev rule to decide wen to *disable* KMS modifier support, as it so that
only the Intel driver has this problem, while on the other hand, there
several drivers that requires modifiers to function at all.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1792>

data/61-mutter.rules

@@ -1 +1 @@
-DRIVER=="tegra-host1x", SUBSYSTEM=="platform", TAG+="mutter-device-requires-kms-modifiers"
+DRIVERS=="i915", SUBSYSTEM=="drm", TAG+="mutter-device-disable-kms-modifiers"

src/backends/native/meta-backend-native.c

@@ -423,8 +423,8 @@ create_gpu_from_udev_device (MetaBackendNative  *native,
   if (meta_is_udev_device_boot_vga (device))
     flags |= META_KMS_DEVICE_FLAG_BOOT_VGA;
 
-  if (meta_is_udev_device_requires_modifiers (device))
-    flags |= META_KMS_DEVICE_FLAG_REQUIRES_MODIFIERS;
+  if (meta_is_udev_device_disable_modifiers (device))
+    flags |= META_KMS_DEVICE_FLAG_DISABLE_MODIFIERS;
 
   if (meta_is_udev_device_preferred_primary (device))
     flags |= META_KMS_DEVICE_FLAG_PREFERRED_PRIMARY;

src/backends/native/meta-gpu-kms.c

@@ -149,12 +149,12 @@ meta_gpu_kms_is_platform_device (MetaGpuKms *gpu_kms)
 }
 
 gboolean
-meta_gpu_kms_requires_modifiers (MetaGpuKms *gpu_kms)
+meta_gpu_kms_disable_modifiers (MetaGpuKms *gpu_kms)
 {
   MetaKmsDeviceFlag flags;
 
   flags = meta_kms_device_get_flags (gpu_kms->kms_device);
-  return !!(flags & META_KMS_DEVICE_FLAG_REQUIRES_MODIFIERS);
+  return !!(flags & META_KMS_DEVICE_FLAG_DISABLE_MODIFIERS);
 }
 
 static int

src/backends/native/meta-gpu-kms.h

@@ -47,7 +47,7 @@ gboolean meta_gpu_kms_is_crtc_active (MetaGpuKms *gpu_kms,
 
 gboolean meta_gpu_kms_is_boot_vga (MetaGpuKms *gpu_kms);
 gboolean meta_gpu_kms_is_platform_device (MetaGpuKms *gpu_kms);
-gboolean meta_gpu_kms_requires_modifiers (MetaGpuKms *gpu_kms);
+gboolean meta_gpu_kms_disable_modifiers (MetaGpuKms *gpu_kms);
 
 MetaKmsDevice * meta_gpu_kms_get_kms_device (MetaGpuKms *gpu_kms);
 

src/backends/native/meta-kms-types.h

@@ -59,7 +59,7 @@ typedef enum _MetaKmsDeviceFlag
   META_KMS_DEVICE_FLAG_NONE = 0,
   META_KMS_DEVICE_FLAG_BOOT_VGA = 1 << 0,
   META_KMS_DEVICE_FLAG_PLATFORM_DEVICE = 1 << 1,
-  META_KMS_DEVICE_FLAG_REQUIRES_MODIFIERS = 1 << 2,
+  META_KMS_DEVICE_FLAG_DISABLE_MODIFIERS = 1 << 2,
   META_KMS_DEVICE_FLAG_PREFERRED_PRIMARY = 1 << 3,
   META_KMS_DEVICE_FLAG_NO_MODE_SETTING = 1 << 4,
 } MetaKmsDeviceFlag;

src/backends/native/meta-renderer-native.c

@@ -1984,6 +1984,8 @@ meta_renderer_native_initable_init (GInitable     *initable,
   gpus = meta_backend_get_gpus (backend);
   if (gpus)
     {
+      const char *use_kms_modifiers_debug_env;
+
       for (l = gpus; l; l = l->next)
         {
           MetaGpuKms *gpu_kms = META_GPU_KMS (l->data);
@@ -1998,8 +2000,20 @@ meta_renderer_native_initable_init (GInitable     *initable,
       if (!renderer_native->primary_gpu_kms)
         return FALSE;
 
-      if (meta_gpu_kms_requires_modifiers (renderer_native->primary_gpu_kms))
-        renderer_native->use_modifiers = TRUE;
+      use_kms_modifiers_debug_env = g_getenv ("MUTTER_DEBUG_USE_KMS_MODIFIERS");
+      if (use_kms_modifiers_debug_env)
+        {
+          renderer_native->use_modifiers =
+            g_strcmp0 (use_kms_modifiers_debug_env, "1") == 0;
+        }
+      else
+        {
+          renderer_native->use_modifiers =
+            !meta_gpu_kms_disable_modifiers (renderer_native->primary_gpu_kms);
+        }
+
+      meta_topic (META_DEBUG_KMS, "Usage of KMS modifiers is %s",
+                  renderer_native->use_modifiers ? "enabled" : "disabled");
     }
   else
     {

src/backends/native/meta-udev.c

@@ -75,7 +75,7 @@ meta_is_udev_device_boot_vga (GUdevDevice *device)
 }
 
 gboolean
-meta_is_udev_device_requires_modifiers (GUdevDevice *device)
+meta_is_udev_device_disable_modifiers (GUdevDevice *device)
 {
   g_autoptr (GUdevDevice) platform_device = NULL;
   const char * const * tags;
@@ -92,7 +92,7 @@ meta_is_udev_device_requires_modifiers (GUdevDevice *device)
   if (!tags)
     return FALSE;
 
-  return g_strv_contains (tags, "mutter-device-requires-kms-modifiers");
+  return g_strv_contains (tags, "mutter-device-disable-kms-modifiers");
 }
 
 gboolean

src/backends/native/meta-udev.h

@@ -32,7 +32,7 @@ gboolean meta_is_udev_device_platform_device (GUdevDevice *device);
 
 gboolean meta_is_udev_device_boot_vga (GUdevDevice *device);
 
-gboolean meta_is_udev_device_requires_modifiers (GUdevDevice *device);
+gboolean meta_is_udev_device_disable_modifiers (GUdevDevice *device);
 
 gboolean meta_is_udev_device_preferred_primary (GUdevDevice *device);