backend/drm: add WLR_DRM_NO_ATOMIC_GAMMA env variable

This allows the legacy interface to be used instead of atomic if needed. This
is a workaround to make some Intel GPUs work (if this variable is unset) as
well as to make some AMD GPUs work (if this variable is set).

backend/drm/atomic.c

@@ -208,10 +208,12 @@ static bool atomic_crtc_set_gamma(struct wlr_drm_backend *drm,
 		struct wlr_drm_crtc *crtc, size_t size,
 		uint16_t *r, uint16_t *g, uint16_t *b) {
 	// Fallback to legacy gamma interface when gamma properties are not available
-	// (can happen on older intel gpu's that support gamma but not degamma)
-	// TEMP: This is broken on AMDGPU. Always fallback to legacy until they get
-	// it fixed. Ref https://bugs.freedesktop.org/show_bug.cgi?id=107459
-	if (crtc->props.gamma_lut == 0 || true) {
+	// (can happen on older Intel GPUs that support gamma but not degamma).
+	// TEMP: This is broken on AMDGPU. Provide a fallback to legacy until they
+	// get it fixed. Ref https://bugs.freedesktop.org/show_bug.cgi?id=107459
+	const char *no_atomic_str = getenv("WLR_DRM_NO_ATOMIC_GAMMA");
+	bool no_atomic = no_atomic_str != NULL && strcmp(no_atomic_str, "1") == 0;
+	if (crtc->props.gamma_lut == 0 || no_atomic) {
 		return legacy_iface.crtc_set_gamma(drm, crtc, size, r, g, b);
 	}
 

docs/env_vars.md

@@ -7,6 +7,8 @@ wlroots specific
   considered the primary DRM device.
 * *WLR_DRM_NO_ATOMIC*: set to 1 to use legacy DRM interface instead of atomic
   mode setting
+* *WLR_DRM_NO_ATOMIC_GAMMA*: set to 1 to use legacy DRM interface for gamma
+  control instead of the atomic interface
 * *WLR_LIBINPUT_NO_DEVICES*: set to 1 to not fail without any input devices
 * *WLR_BACKENDS*: comma-separated list of backends to use (available backends:
   wayland, x11, headless)