diff --git a/firmware/include/gbb_header.h b/firmware/include/gbb_header.h
index 43aa3e7..86aa12a 100644
--- a/firmware/include/gbb_header.h
+++ b/firmware/include/gbb_header.h
@@ -58,6 +58,10 @@
 #define GBB_FLAG_FORCE_DEV_BOOT_LEGACY    0x00000080
 /* Allow booting using alternate keys for FAFT servo testing */
 #define GBB_FLAG_FAFT_KEY_OVERIDE         0x00000100
+/* Disable EC software sync */
+#define GBB_FLAG_DISABLE_EC_SOFTWARE_SYNC 0x00000200
+/* Default to booting legacy OS when dev screen times out */
+#define GBB_FLAG_DEFAULT_DEV_BOOT_LEGACY  0x00000400
 
 #ifdef __cplusplus
 extern "C" {
diff --git a/firmware/lib/vboot_api_kernel.c b/firmware/lib/vboot_api_kernel.c
index a1d83b1..64f600a 100644
--- a/firmware/lib/vboot_api_kernel.c
+++ b/firmware/lib/vboot_api_kernel.c
@@ -177,7 +177,7 @@
 		(GoogleBinaryBlockHeader *)cparams->gbb_data;
 	VbSharedDataHeader *shared =
 		(VbSharedDataHeader *)cparams->shared_data_blob;
-	uint32_t allow_usb = 0, allow_legacy = 0;
+	uint32_t allow_usb = 0, allow_legacy = 0, ctrl_d_pressed = 0;
 	VbAudioContext *audio = 0;
 
 	VBDEBUG(("Entering %s()\n", __func__));
@@ -287,6 +287,7 @@
 			/* Ctrl+D = dismiss warning; advance to timeout */
 			VBDEBUG(("VbBootDeveloper() - "
 				 "user pressed Ctrl+D; skip delay\n"));
+			ctrl_d_pressed = 1;
 			goto fallout;
 			break;
 		case 0x0c:
@@ -370,6 +371,19 @@
 	} while(VbAudioLooping(audio));
 
  fallout:
+
+	/* If defaulting to legacy boot, try that unless Ctrl+D was pressed */
+	if ((gbb->flags & GBB_FLAG_DEFAULT_DEV_BOOT_LEGACY) &&
+	    !ctrl_d_pressed) {
+		VBDEBUG(("VbBootDeveloper() - defaulting to legacy\n"));
+		VbExLegacy();
+
+		/* If that fails, beep and fall through to fixed disk */
+		VbExBeep(120, 400);
+		VbExSleepMs(120);
+		VbExBeep(120, 400);
+	}
+
 	/* Timeout or Ctrl+D; attempt loading from fixed disk */
 	VBDEBUG(("VbBootDeveloper() - trying fixed disk\n"));
 	VbAudioClose(audio);
@@ -798,6 +812,8 @@
 {
 	VbSharedDataHeader *shared =
 		(VbSharedDataHeader *)cparams->shared_data_blob;
+	GoogleBinaryBlockHeader *gbb =
+		(GoogleBinaryBlockHeader *)cparams->gbb_data;
 	VbError_t retval = VBERROR_SUCCESS;
 	LoadKernelParams p;
 	uint32_t tpm_status = 0;
@@ -816,7 +832,8 @@
 	Memset(kparams->partition_guid, 0, sizeof(kparams->partition_guid));
 
 	/* Do EC software sync if necessary */
-	if (shared->flags & VBSD_EC_SOFTWARE_SYNC) {
+	if ((shared->flags & VBSD_EC_SOFTWARE_SYNC) &&
+	    !(gbb->flags & GBB_FLAG_DISABLE_EC_SOFTWARE_SYNC)) {
 		retval = VbEcSoftwareSync(cparams);
 		if (retval != VBERROR_SUCCESS)
 			goto VbSelectAndLoadKernel_exit;
diff --git a/scripts/image_signing/set_gbb_flags.sh b/scripts/image_signing/set_gbb_flags.sh
index ac52432..5469f1a 100755
--- a/scripts/image_signing/set_gbb_flags.sh
+++ b/scripts/image_signing/set_gbb_flags.sh
@@ -31,6 +31,8 @@
   GBB_FLAG_ENTER_TRIGGERS_TONORM      0x00000040
   GBB_FLAG_FORCE_DEV_BOOT_LEGACY      0x00000080
   GBB_FLAG_FAFT_KEY_OVERIDE           0x00000100
+  GBB_FLAG_DISABLE_EC_SOFTWARE_SYNC   0x00000200
+  GBB_FLAG_DEFAULT_DEV_BOOT_LEGACY    0x00000400
 
   To get a developer-friendly device, try 0x11 (short_delay + boot_usb).
   For factory-related tests (always DEV), try 0x39.
diff --git a/tests/vboot_api_kernel2_tests.c b/tests/vboot_api_kernel2_tests.c
index aa3e20d..c1572c2 100644
--- a/tests/vboot_api_kernel2_tests.c
+++ b/tests/vboot_api_kernel2_tests.c
@@ -220,6 +220,12 @@
 	TEST_EQ(u, 0, "  recovery reason");
 	TEST_EQ(audio_looping_calls_left, 0, "  used up audio");
 
+	/* Proceed to legacy after timeout if GBB flag set */
+	ResetMocks();
+	gbb.flags |= GBB_FLAG_DEFAULT_DEV_BOOT_LEGACY;
+	TEST_EQ(VbBootDeveloper(&cparams, &lkp), 1002, "Timeout");
+	TEST_EQ(vbexlegacy_called, 1, "  try legacy");
+
 	/* Up arrow is uninteresting / passed to VbCheckDisplayKey() */
 	ResetMocks();
 	mock_keypress[0] = VB_KEY_UP;
@@ -313,6 +319,14 @@
 	VbNvGet(VbApiKernelGetVnc(), VBNV_RECOVERY_REQUEST, &u);
 	TEST_EQ(u, 0, "  recovery reason");
 	TEST_NEQ(audio_looping_calls_left, 0, "  aborts audio");
+	TEST_EQ(vbexlegacy_called, 0, "  not legacy");
+
+	/* Ctrl+D doesn't boot legacy even if GBB flag is set */
+	ResetMocks();
+	mock_keypress[0] = 0x04;
+	gbb.flags |= GBB_FLAG_DEFAULT_DEV_BOOT_LEGACY;
+	TEST_EQ(VbBootDeveloper(&cparams, &lkp), 1002, "Ctrl+D");
+	TEST_EQ(vbexlegacy_called, 0, "  not legacy");
 
 	/* Ctrl+L tries legacy boot mode only if enabled */
 	ResetMocks();
diff --git a/tests/vboot_api_kernel4_tests.c b/tests/vboot_api_kernel4_tests.c
index af8d2b6..81628a4 100644
--- a/tests/vboot_api_kernel4_tests.c
+++ b/tests/vboot_api_kernel4_tests.c
@@ -155,6 +155,12 @@
 	ecsync_retval = VBERROR_SIMULATED;
 	test_slk(0, 0, "EC sync not done");
 
+	ResetMocks();
+	shared->flags |= VBSD_EC_SOFTWARE_SYNC;
+	gbb.flags |= GBB_FLAG_DISABLE_EC_SOFTWARE_SYNC;
+	ecsync_retval = VBERROR_SIMULATED;
+	test_slk(0, 0, "EC sync disabled by GBB");
+
 	/* Rollback kernel version */
 	ResetMocks();
 	rkr_retval = 123;