Keep the HMD alive while waiting for camera enumeration

src/xrt/drivers/rift_s/rift_s.c

@@ -246,6 +246,7 @@ rift_s_system_create(struct xrt_prober *xp,
 		RIFT_S_ERROR("Failed to enable Rift S");
 		goto cleanup;
 	}
+	rift_s_send_keepalive(sys->handles[HMD_HID]);
 
 	// Bench sessions must power the panel even if no new proximity packet arrives.
 	if (debug_get_bool_option_steamvr_bench()) {
@@ -860,6 +861,7 @@ open_transport(struct rift_s_system *sys)
 	    rift_s_hmd_enable(handles[HMD_HID], true) < 0 ||
 	    rift_s_set_screen_enable(handles[HMD_HID], true) < 0)
 		goto fail;
+	rift_s_send_keepalive(handles[HMD_HID]);
 	// Power the panel during enumeration even off-head. Proximity packets restore
 	// ordinary standby after the DP link and camera input have returned.
 	os_nanosleep(2ULL * U_TIME_1S_IN_NS);

src/xrt/drivers/rift_s/rift_s_camera.c

@@ -151,6 +151,8 @@ rift_s_camera_create(struct xrt_prober *xp,
 	/* Re-probe devices. The v4l2 camera device should have appeared by now */
 	int retry_count = 5;
 	do {
+		// Display/camera enumeration can outlast the firmware's three-second lease.
+		rift_s_send_keepalive(hid);
 		xrt_result_t xret = xrt_prober_probe(xp);
 
 		if (xret != XRT_SUCCESS) {