tree: Enable warning for fallthrough in switch statements

The EC code has generally been good about adding comments about
intentional fallthrough in switch statements, but there were a few cases
without comments (e.g., https://crrev.com/c/3949622). Enabling
-Wimplicit-fallthrough generates a compiler warning if the fallthrough
is not annotated with __attribute__((fallthrough)). For convenience, we
add a "__fallthrough" macro for this attribute.

See https://clang.llvm.org/docs/AttributeReference.html#fallthrough and
https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html.

BRANCH=none
BUG=b:253644823
TEST=make buildall
LOW_COVERAGE_REASON=legacy code

Signed-off-by: Tom Hughes <tomhughes@chromium.org>
Change-Id: I9f4d6049f4507a25ce706675d159b70e28b4b825
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3957420
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
Code-Coverage: Zoss <zoss-cl-coverage@prod.google.com>

53 files changed, 86 insertions, 44 deletions

diff --git a/Makefile.toolchain b/Makefile.toolchain
index 3c93d774d4..c2612d260c 100644
--- a/Makefile.toolchain
+++ b/Makefile.toolchain
@@ -115,7 +115,7 @@ C_WARN = -Wstrict-prototypes -Wdeclaration-after-statement -Wno-pointer-sign
 C_WARN += -Werror-implicit-function-declaration
 COMMON_WARN = -Wall -Wundef \
 	-Wno-trigraphs -Wno-format-security -Wno-address-of-packed-member \
-	-fno-common -fno-strict-aliasing -fno-strict-overflow
+	-fno-common -fno-strict-aliasing -fno-strict-overflow -Wimplicit-fallthrough
 # See https://www.chromium.org/chromium-os/build/c-exception-support
 COMMON_WARN+=-fno-exceptions -fno-unwind-tables -fno-asynchronous-unwind-tables
 
diff --git a/baseboard/intelrvp/led_states.c b/baseboard/intelrvp/led_states.c
index 8629085c4f..6bcee159a3 100644
--- a/baseboard/intelrvp/led_states.c
+++ b/baseboard/intelrvp/led_states.c
@@ -39,7 +39,7 @@ static enum led_states led_get_state(void)
 			new_state = STATE_CHARGING_FULL_CHARGE;
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE /* and PWR_STATE_DISCHARGE_FULL */:
 		if (chipset_in_state(CHIPSET_STATE_ON))
 			new_state = STATE_DISCHARGE_S0;
diff --git a/board/boldar/board.c b/board/boldar/board.c
index 19b8f44020..1076ccc040 100644
--- a/board/boldar/board.c
+++ b/board/boldar/board.c
@@ -474,6 +474,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/burnet/led.c b/board/burnet/led.c
index 4aa91c63e2..68c4c9e839 100644
--- a/board/burnet/led.c
+++ b/board/burnet/led.c
@@ -147,7 +147,7 @@ static void led_set_battery(void)
 			led_set_color_battery(LED_WHITE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE:
 		/*
 		 * Blink white light (1 sec on, 1 sec off)
diff --git a/board/careena/led.c b/board/careena/led.c
index 48a065fdb9..9a280a011b 100644
--- a/board/careena/led.c
+++ b/board/careena/led.c
@@ -110,7 +110,7 @@ static void led_set_battery(void)
 			led_set_color_battery(LED_WHITE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE:
 		/*
 		 * Blink white light (1 sec on, 1 sec off)
diff --git a/board/chronicler/usbc_config.c b/board/chronicler/usbc_config.c
index b05cd4ecfd..85c40e93e1 100644
--- a/board/chronicler/usbc_config.c
+++ b/board/chronicler/usbc_config.c
@@ -68,6 +68,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/collis/board.c b/board/collis/board.c
index 6b2c81bc02..56c67989a5 100644
--- a/board/collis/board.c
+++ b/board/collis/board.c
@@ -270,6 +270,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/copano/board.c b/board/copano/board.c
index b4a8c5eb20..8d58fa1101 100644
--- a/board/copano/board.c
+++ b/board/copano/board.c
@@ -310,6 +310,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/coral/led.c b/board/coral/led.c
index 8af0f6f3e2..b8515c14c7 100644
--- a/board/coral/led.c
+++ b/board/coral/led.c
@@ -188,7 +188,7 @@ static enum led_states led_get_state(void)
 			new_state = STATE_CHARGING_LVL_3;
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE /* and PWR_STATE_DISCHARGE_FULL */:
 		if (chipset_in_state(CHIPSET_STATE_ON))
 			new_state = STATE_DISCHARGE_S0;
diff --git a/board/corori/led.c b/board/corori/led.c
index 89799c8e74..f47871a4ed 100644
--- a/board/corori/led.c
+++ b/board/corori/led.c
@@ -127,7 +127,7 @@ static enum led_states led_get_state(void)
 			new_state = STATE_CHARGING_FULL_CHARGE;
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE /* and PWR_STATE_DISCHARGE_FULL */:
 		if (chipset_in_state(CHIPSET_STATE_ON))
 			new_state = (led_get_charge_percent() < 10) ?
diff --git a/board/delbin/board.c b/board/delbin/board.c
index 1cdb42d954..28a0c929e2 100644
--- a/board/delbin/board.c
+++ b/board/delbin/board.c
@@ -370,6 +370,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/drawcia/led.c b/board/drawcia/led.c
index 8e6263fbbc..0fee248506 100644
--- a/board/drawcia/led.c
+++ b/board/drawcia/led.c
@@ -142,7 +142,7 @@ static void led_set_battery(void)
 			led_set_color_battery(LED_WHITE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE:
 		/*
 		 * Blink white light (1 sec on, 1 sec off)
diff --git a/board/drawcia_riscv/led.c b/board/drawcia_riscv/led.c
index 3cb7cb835e..d72626b2a2 100644
--- a/board/drawcia_riscv/led.c
+++ b/board/drawcia_riscv/led.c
@@ -142,7 +142,7 @@ static void led_set_battery(void)
 			led_set_color_battery(LED_WHITE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE:
 		/*
 		 * Blink white light (1 sec on, 1 sec off)
diff --git a/board/drobit/board.c b/board/drobit/board.c
index 19b5162092..1a20870d1f 100644
--- a/board/drobit/board.c
+++ b/board/drobit/board.c
@@ -303,6 +303,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/eldrid/board.c b/board/eldrid/board.c
index 717878bc95..81fdf22ffc 100644
--- a/board/eldrid/board.c
+++ b/board/eldrid/board.c
@@ -516,6 +516,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/elemi/board.c b/board/elemi/board.c
index 6b166e1206..96860d2335 100644
--- a/board/elemi/board.c
+++ b/board/elemi/board.c
@@ -347,6 +347,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/haboki/led.c b/board/haboki/led.c
index b31d4fab75..c6e481e285 100644
--- a/board/haboki/led.c
+++ b/board/haboki/led.c
@@ -142,7 +142,7 @@ static void led_set_battery(void)
 			led_set_color_battery(LED_WHITE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE:
 		/*
 		 * Blink white light (1 sec on, 1 sec off)
diff --git a/board/kappa/led.c b/board/kappa/led.c
index 1c7979eac0..1dc86013d7 100644
--- a/board/kappa/led.c
+++ b/board/kappa/led.c
@@ -104,7 +104,7 @@ static void led_set_battery(void)
 			led_set_color_battery(LED_WHITE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE:
 		/*
 		 * Blink white light (1 sec on, 1 sec off)
diff --git a/board/lantis/led.c b/board/lantis/led.c
index d6dbc929a8..3b66ad6d6b 100644
--- a/board/lantis/led.c
+++ b/board/lantis/led.c
@@ -199,7 +199,7 @@ static void led_set_battery(void)
 			set_active_port_color(LED_WHITE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE:
 		/*
 		 * Blink white/amber light (1 sec on, 1 sec off)
diff --git a/board/lindar/board.c b/board/lindar/board.c
index 5bb155550e..e3ee0ff9c4 100644
--- a/board/lindar/board.c
+++ b/board/lindar/board.c
@@ -479,6 +479,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/reef/led.c b/board/reef/led.c
index 95db062d10..8bc52dbc75 100644
--- a/board/reef/led.c
+++ b/board/reef/led.c
@@ -104,7 +104,7 @@ static void led_set_battery(void)
 			led_set_color_battery(LED_BLUE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE /* and PWR_STATE_DISCHARGE_FULL */:
 		if (chipset_in_state(CHIPSET_STATE_ON)) {
 			led_set_color_battery(LED_BLUE);
diff --git a/board/reef_it8320/led.c b/board/reef_it8320/led.c
index 1b521b9409..630fdf336d 100644
--- a/board/reef_it8320/led.c
+++ b/board/reef_it8320/led.c
@@ -105,7 +105,7 @@ static void led_set_battery(void)
 			led_set_color_battery(LED_BLUE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE /* and PWR_STATE_DISCHARGE_FULL */:
 		if (chipset_in_state(CHIPSET_STATE_ON)) {
 			led_set_color_battery(LED_BLUE);
diff --git a/board/reef_mchp/led.c b/board/reef_mchp/led.c
index 469758dc26..96ea6bb958 100644
--- a/board/reef_mchp/led.c
+++ b/board/reef_mchp/led.c
@@ -105,7 +105,7 @@ static void led_set_battery(void)
 			led_set_color_battery(LED_BLUE);
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE /* and PWR_STATE_DISCHARGE_FULL */:
 		if (chipset_in_state(CHIPSET_STATE_ON)) {
 			led_set_color_battery(LED_BLUE);
diff --git a/board/voema/board.c b/board/voema/board.c
index c4563df105..363cc603ad 100644
--- a/board/voema/board.c
+++ b/board/voema/board.c
@@ -309,6 +309,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/volet/board.c b/board/volet/board.c
index 474ec9cb0e..47140464b6 100644
--- a/board/volet/board.c
+++ b/board/volet/board.c
@@ -369,6 +369,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/board/voxel/board.c b/board/voxel/board.c
index 40dfbe8dc4..4c8badeb1c 100644
--- a/board/voxel/board.c
+++ b/board/voxel/board.c
@@ -367,6 +367,7 @@ void ppc_interrupt(enum gpio_signal signal)
 		break;
 	case GPIO_USB_C1_PPC_INT_ODL:
 		syv682x_interrupt(USBC_PORT_C1);
+		break;
 	default:
 		break;
 	}
diff --git a/chip/npcx/system.c b/chip/npcx/system.c
index 958d873401..9ce043354f 100644
--- a/chip/npcx/system.c
+++ b/chip/npcx/system.c
@@ -1410,6 +1410,7 @@ void system_set_image_copy(enum ec_image copy)
 			"Change it to %d.",
 			copy, EC_IMAGE_RO);
 		/* Fall through to EC_IMAGE_RO */
+		__fallthrough;
 	case EC_IMAGE_RO:
 		SET_BIT(NPCX_FWCTRL, NPCX_FWCTRL_RO_REGION);
 		SET_BIT(NPCX_FWCTRL, NPCX_FWCTRL_FW_SLOT);
diff --git a/chip/npcx/wov.c b/chip/npcx/wov.c
index 1ae1afe71d..8616cdd001 100644
--- a/chip/npcx/wov.c
+++ b/chip/npcx/wov.c
@@ -836,6 +836,7 @@ enum ec_error_list wov_set_mode(enum wov_modes wov_mode)
 	case WOV_MODE_RAM_AND_I2S:
 		if ((wov_conf.bit_depth != 16) && (wov_conf.bit_depth != 24))
 			return EC_ERROR_INVAL;
+		__fallthrough;
 	case WOV_MODE_I2S:
 		if (apm_get_adc_i2s_dmic_rate() == APM_DMIC_RATE_0_75)
 			wov_dmic_clk_config(1, WOV_DMIC_DIV_BY_4);
diff --git a/common/charge_ramp_sw.c b/common/charge_ramp_sw.c
index 4c09a94a8d..4e5bdc8149 100644
--- a/common/charge_ramp_sw.c
+++ b/common/charge_ramp_sw.c
@@ -214,6 +214,7 @@ void chg_ramp_task(void *u)
 			ramp_st_new = CHG_RAMP_OVERCURRENT_DETECT;
 			/* notify host of power info change */
 			pd_send_host_event(PD_EVENT_POWER_CHANGE);
+			__fallthrough;
 		case CHG_RAMP_OVERCURRENT_DETECT:
 			/* Check if we should ramp or go straight to stable */
 			task_wait_time = SECOND;
diff --git a/common/console.c b/common/console.c
index 9e7d6ad036..b045af1ddf 100644
--- a/common/console.c
+++ b/common/console.c
@@ -507,6 +507,7 @@ static void console_handle_char(int c)
 		move_cursor_right();
 
 		/* Drop through to backspace handling */
+		__fallthrough;
 	case '\b':
 	case 0x7f:
 		handle_backspace();
diff --git a/common/led_onoff_states.c b/common/led_onoff_states.c
index 244257cf1d..404ae06cd9 100644
--- a/common/led_onoff_states.c
+++ b/common/led_onoff_states.c
@@ -71,7 +71,7 @@ static enum led_states led_get_state(void)
 				new_state = STATE_CHARGING_FULL_CHARGE;
 			break;
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case PWR_STATE_DISCHARGE /* and PWR_STATE_DISCHARGE_FULL */:
 		if (chipset_in_state(CHIPSET_STATE_ON)) {
 #ifdef CONFIG_LED_ONOFF_STATES_BAT_LOW
diff --git a/common/motion_sense.c b/common/motion_sense.c
index f326259497..38bc5ab702 100644
--- a/common/motion_sense.c
+++ b/common/motion_sense.c
@@ -1320,7 +1320,7 @@ static enum ec_status host_cmd_motion_sense(struct host_cmd_handler_args *args)
 
 		task_set_event(TASK_ID_MOTIONSENSE,
 			       TASK_EVENT_MOTION_FLUSH_PENDING);
-		/* pass-through */
+		__fallthrough;
 	case MOTIONSENSE_CMD_FIFO_INFO:
 		if (!IS_ENABLED(CONFIG_ACCEL_FIFO)) {
 			/*
@@ -1352,7 +1352,7 @@ static enum ec_status host_cmd_motion_sense(struct host_cmd_handler_args *args)
 		case 0:
 		case 1:
 			fifo_int_enabled = in->fifo_int_enable.enable;
-			/* fallthrough */
+			__fallthrough;
 		case EC_MOTION_SENSE_NO_VALUE:
 			out->fifo_int_enable.ret = fifo_int_enabled;
 			args->response_size = sizeof(out->fifo_int_enable);
diff --git a/common/system.c b/common/system.c
index c3a0f8ac58..6f64960170 100644
--- a/common/system.c
+++ b/common/system.c
@@ -1020,7 +1020,7 @@ static int handle_pending_reboot(enum ec_reboot_cmd cmd)
 			chip_save_reset_flags(chip_read_reset_flags() &
 					      ~EC_RESET_FLAG_AP_IDLE);
 		}
-		/* Intentional fall-through */
+		__fallthrough;
 	case EC_REBOOT_HIBERNATE:
 		if (!IS_ENABLED(CONFIG_HIBERNATE))
 			return EC_ERROR_INVAL;
diff --git a/common/usb_port_power_dumb.c b/common/usb_port_power_dumb.c
index 10eca3ffb7..cfdc016662 100644
--- a/common/usb_port_power_dumb.c
+++ b/common/usb_port_power_dumb.c
@@ -104,7 +104,7 @@ static int command_set_mode(int argc, const char **argv)
 			return EC_ERROR_PARAM2;
 
 		usb_port_set_enabled(port_id, mode);
-		/* fallthrough */
+		__fallthrough;
 	case 1:
 		for (i = 0; i < USB_PORT_COUNT; i++)
 			ccprintf("Port %d: %s\n", i,
diff --git a/common/usbc/dp_alt_mode.c b/common/usbc/dp_alt_mode.c
index 0f2a42ec2a..f53d212b91 100644
--- a/common/usbc/dp_alt_mode.c
+++ b/common/usbc/dp_alt_mode.c
@@ -283,6 +283,7 @@ enum dpm_msg_setup_status dp_setup_next_vdm(int port, int *vdo_count,
 			return MSG_SETUP_MUX_WAIT;
 		}
 		/* Fall through if no mux set is needed */
+		__fallthrough;
 	case DP_PREPARE_CONFIG:
 		vdo_count_ret = modep->fx->config(port, vdm);
 		if (vdo_count_ret == 0)
@@ -293,6 +294,7 @@ enum dpm_msg_setup_status dp_setup_next_vdm(int port, int *vdo_count,
 	case DP_ENTER_NAKED:
 		DP_SET_FLAG(port, DP_FLAG_RETRY);
 		/* Fall through to send exit mode */
+		__fallthrough;
 	case DP_ACTIVE:
 		/*
 		 * Called to exit DP alt mode, either when the mode
diff --git a/common/usbc/tbt_alt_mode.c b/common/usbc/tbt_alt_mode.c
index d1ad031fad..c0e4e172cc 100644
--- a/common/usbc/tbt_alt_mode.c
+++ b/common/usbc/tbt_alt_mode.c
@@ -514,7 +514,7 @@ enum dpm_msg_setup_status tbt_setup_next_vdm(int port, int *vdo_count,
 		 * done with any retires.
 		 */
 		TBT_SET_FLAG(port, TBT_FLAG_RETRY_DONE);
-		/* Fall through */
+		__fallthrough;
 	case TBT_PREPARE_EXIT_MODE:
 		/*
 		 * Called to exit Thunderbolt alt mode, either when the mode is
diff --git a/common/usbc/usb_pd_dpm.c b/common/usbc/usb_pd_dpm.c
index 3af3ac3dbe..2538ef0be6 100644
--- a/common/usbc/usb_pd_dpm.c
+++ b/common/usbc/usb_pd_dpm.c
@@ -243,6 +243,7 @@ void dpm_vdm_acked(int port, enum tcpci_msg_type type, int vdo_count,
 			intel_vdm_acked(port, type, vdo_count, vdm);
 			break;
 		}
+		__fallthrough;
 	default:
 		CPRINTS("C%d: Received unexpected VDM ACK for SVID %d", port,
 			svid);
@@ -261,6 +262,7 @@ void dpm_vdm_naked(int port, enum tcpci_msg_type type, uint16_t svid,
 			intel_vdm_naked(port, type, vdm_cmd);
 			break;
 		}
+		__fallthrough;
 	default:
 		CPRINTS("C%d: Received unexpected VDM NAK for SVID %d", port,
 			svid);
diff --git a/common/usbc/usb_pe_drp_sm.c b/common/usbc/usb_pe_drp_sm.c
index 2e3bffbf68..5a5aebc4b4 100644
--- a/common/usbc/usb_pe_drp_sm.c
+++ b/common/usbc/usb_pe_drp_sm.c
@@ -817,11 +817,11 @@ void pe_run(int port, int evt, int en)
 	case SM_PAUSED:
 		if (!en)
 			break;
-		/* fall through */
+		__fallthrough;
 	case SM_INIT:
 		pe_init(port);
 		local_state[port] = SM_RUN;
-		/* fall through */
+		__fallthrough;
 	case SM_RUN:
 		if (!en) {
 			local_state[port] = SM_PAUSED;
@@ -5997,8 +5997,8 @@ static void pe_init_vdm_modes_request_run(int port)
 			dfp_consume_modes(port, sop, cnt, payload);
 			break;
 		}
+		__fallthrough;
 	}
-		/* Fall Through */
 	case VDM_RESULT_NAK:
 		/* PE_INIT_VDM_Modes_NAKed embedded here */
 		pd_set_modes_discovery(port, pe[port].tx_type, requested_svid,
diff --git a/common/usbc/usb_prl_sm.c b/common/usbc/usb_prl_sm.c
index d4bd5b9a6e..db9f460e61 100644
--- a/common/usbc/usb_prl_sm.c
+++ b/common/usbc/usb_prl_sm.c
@@ -696,11 +696,11 @@ void prl_run(int port, int evt, int en)
 	case SM_PAUSED:
 		if (!en)
 			break;
-		/* fall through */
+		__fallthrough;
 	case SM_INIT:
 		prl_init(port);
 		local_state[port] = SM_RUN;
-		/* fall through */
+		__fallthrough;
 	case SM_RUN:
 		if (!en) {
 			/* Disable RX */
diff --git a/common/usbc/usb_retimer_fw_update.c b/common/usbc/usb_retimer_fw_update.c
index 3f9b1b4c72..9b581ea872 100644
--- a/common/usbc/usb_retimer_fw_update.c
+++ b/common/usbc/usb_retimer_fw_update.c
@@ -116,7 +116,7 @@ int usb_retimer_fw_update_get_result(void)
 			result = last_result;
 			break;
 		}
-		/* fall through */
+		__fallthrough;
 	case USB_RETIMER_FW_UPDATE_RESUME_PD:
 		result = pd_is_port_enabled(cur_port);
 		break;
diff --git a/driver/als_si114x.c b/driver/als_si114x.c
index 30f92e23d1..eba613be8b 100644
--- a/driver/als_si114x.c
+++ b/driver/als_si114x.c
@@ -317,10 +317,13 @@ static int si114x_set_chlist(const struct motion_sensor_t *s)
 	switch (SI114X_NUM_LEDS) {
 	case 3:
 		reg |= SI114X_PARAM_CHLIST_EN_PS3;
+		__fallthrough;
 	case 2:
 		reg |= SI114X_PARAM_CHLIST_EN_PS2;
+		__fallthrough;
 	case 1:
 		reg |= SI114X_PARAM_CHLIST_EN_PS1;
+		__fallthrough;
 	case 0:
 		break;
 	}
diff --git a/driver/charger/bd9995x.c b/driver/charger/bd9995x.c
index a6303d41b4..aa37bf4e8a 100644
--- a/driver/charger/bd9995x.c
+++ b/driver/charger/bd9995x.c
@@ -758,6 +758,7 @@ static enum ec_error_list bd9995x_get_status(int chgnum, int *status)
 		break;
 	case BD9995X_CMD_CHGOP_STATUS_BATTEMP_BATOPEN:
 		*status &= ~CHARGER_BATTERY_PRESENT;
+		break;
 	default:
 		break;
 	}
diff --git a/driver/charger/rt946x.c b/driver/charger/rt946x.c
index acf5dc012d..58b5c1adb4 100644
--- a/driver/charger/rt946x.c
+++ b/driver/charger/rt946x.c
@@ -1233,7 +1233,7 @@ static int rt946x_get_bc12_ilim(int charge_supplier)
 		    IS_ENABLED(CONFIG_CHARGE_RAMP_HW))
 			/* A conservative value to prevent a bad charger. */
 			return RT946X_AICR_TYP2MAX(USB_CHARGER_MAX_CURR_MA);
-		/* fallback */
+		__fallthrough;
 	case CHARGE_SUPPLIER_BC12_CDP:
 		return USB_CHARGER_MAX_CURR_MA;
 	case CHARGE_SUPPLIER_BC12_SDP:
diff --git a/driver/tcpm/rt1718s.c b/driver/tcpm/rt1718s.c
index a009e33323..07c14f7dd5 100644
--- a/driver/tcpm/rt1718s.c
+++ b/driver/tcpm/rt1718s.c
@@ -196,7 +196,7 @@ static int rt1718s_workaround(int port)
 		RETURN_ERROR(rt1718s_update_bits8(
 			port, RT1718S_VCONN_CONTROL_3,
 			RT1718S_VCONN_CONTROL_3_VCONN_OVP_DEG, 0xFF));
-		/* fallthrough */
+		__fallthrough;
 	case RT1718S_DEVICE_ID_ES2:
 		RETURN_ERROR(rt1718s_update_bits8(
 			port, TCPC_REG_FAULT_CTRL,
diff --git a/include/common.h b/include/common.h
index 72553161f5..97dddfc03f 100644
--- a/include/common.h
+++ b/include/common.h
@@ -192,6 +192,15 @@
  */
 #define __warn_unused_result __attribute__((warn_unused_result))
 
+/**
+ * @brief Attribute used to annotate intentional fallthrough between switch
+ * labels.
+ *
+ * See https://clang.llvm.org/docs/AttributeReference.html#fallthrough and
+ * https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html.
+ */
+#define __fallthrough __attribute__((fallthrough))
+
 /*
  * Macros for combining bytes into larger integers. _LE and _BE signify little
  * and big endian versions respectively.
diff --git a/power/common.c b/power/common.c
index c1675d31f2..65c1cf0779 100644
--- a/power/common.c
+++ b/power/common.c
@@ -521,12 +521,12 @@ static enum power_state power_common_state(void)
 		break;
 
 	case POWER_S4:
-		/* fallthrough */
+		__fallthrough;
 	case POWER_S3:
-		/* fallthrough */
+		__fallthrough;
 	case POWER_S0:
 #ifdef CONFIG_POWER_S0IX
-		/* fallthrough */
+		__fallthrough;
 	case POWER_S0ix:
 #endif
 		/* Wait for a message */
diff --git a/power/intel_x86.c b/power/intel_x86.c
index 1c9f2ef8c8..4fa3aa3c8e 100644
--- a/power/intel_x86.c
+++ b/power/intel_x86.c
@@ -381,7 +381,7 @@ enum power_state common_intel_x86_power_handle_state(enum power_state state)
 		return POWER_S4; /* Power down to the next state */
 
 	case POWER_S5S3:
-		/* fallthrough */
+		__fallthrough;
 	case POWER_S4S3:
 		if (!power_has_signals(IN_PGOOD_ALL_CORE)) {
 			/* Required rail went away */
diff --git a/util/stm32mon.cc b/util/stm32mon.cc
index 352158ae73..39837bd3dd 100644
--- a/util/stm32mon.cc
+++ b/util/stm32mon.cc
@@ -633,6 +633,7 @@ int wait_for_ack(int fd)
 			}
 
 			/* Do not break so that it can be handled as junk */
+			__attribute__((fallthrough));
 		default:
 			stat_resp[JUNK_IDX].event_count++;
 			if (mode == MODE_SERIAL)
diff --git a/zephyr/CMakeLists.txt b/zephyr/CMakeLists.txt
index 84f5aeb97f..ae882e5640 100644
--- a/zephyr/CMakeLists.txt
+++ b/zephyr/CMakeLists.txt
@@ -56,6 +56,7 @@ endif()
 # Set extra compiler flags.
 zephyr_cc_option(-mno-unaligned-access)
 zephyr_cc_option(-fno-PIC)
+zephyr_cc_option(-Wimplicit-fallthrough)
 
 if (DEFINED CONFIG_RISCV)
   zephyr_cc_option(-fsanitize=integer-divide-by-zero)
diff --git a/zephyr/emul/emul_sn5s330.c b/zephyr/emul/emul_sn5s330.c
index f957cd9e05..6a520ed195 100644
--- a/zephyr/emul/emul_sn5s330.c
+++ b/zephyr/emul/emul_sn5s330.c
@@ -227,15 +227,15 @@ static int sn5s330_emul_write_byte(const struct emul *emul, int reg,
 	/* Specially check for read-only reg */
 	switch (reg) {
 	case SN5S330_INT_TRIP_RISE_REG1:
-		/* fallthrough */
+		__fallthrough;
 	case SN5S330_INT_TRIP_RISE_REG2:
-		/* fallthrough */
+		__fallthrough;
 	case SN5S330_INT_TRIP_RISE_REG3:
-		/* fallthrough */
+		__fallthrough;
 	case SN5S330_INT_TRIP_FALL_REG1:
-		/* fallthrough */
+		__fallthrough;
 	case SN5S330_INT_TRIP_FALL_REG2:
-		/* fallthrough */
+		__fallthrough;
 	case SN5S330_INT_TRIP_FALL_REG3:
 		reg_to_write = sn5s330_emul_get_reg_ptr(data, reg);
 		/* Clearing any bit deasserts /INT interrupt signal */
@@ -245,15 +245,15 @@ static int sn5s330_emul_write_byte(const struct emul *emul, int reg,
 		*reg_to_write = val;
 		break;
 	case SN5S330_INT_STATUS_REG1:
-		/* fallthrough */
+		__fallthrough;
 	case SN5S330_INT_STATUS_REG2:
-		/* fallthrough */
+		__fallthrough;
 	case SN5S330_INT_STATUS_REG3:
 		__ASSERT(false,
 			 "Write to an unverified-as-safe read-only register on "
 			 "0x%x",
 			 reg);
-		/* fallthrough for checkpath */
+		__fallthrough;
 	default:
 		reg_to_write = sn5s330_emul_get_reg_ptr(data, reg);
 		*reg_to_write = val;
diff --git a/zephyr/emul/tcpc/emul_ps8xxx.c b/zephyr/emul/tcpc/emul_ps8xxx.c
index 22972bf084..57cc2c85f5 100644
--- a/zephyr/emul/tcpc/emul_ps8xxx.c
+++ b/zephyr/emul/tcpc/emul_ps8xxx.c
@@ -235,6 +235,7 @@ static int ps8xxx_emul_tcpc_write_byte(const struct emul *emul, int reg,
 		if (prod_id != PS8815_PRODUCT_ID) {
 			break;
 		}
+		__fallthrough;
 	case PS8XXX_REG_I2C_DEBUGGING_ENABLE:
 	case PS8XXX_REG_MUX_IN_HPD_ASSERTION:
 	case PS8XXX_REG_BIST_CONT_MODE_BYTE0:
@@ -282,6 +283,7 @@ static int ps8xxx_emul_tcpc_finish_write(const struct emul *emul, int reg,
 		if (prod_id != PS8815_PRODUCT_ID) {
 			break;
 		}
+		__fallthrough;
 	case PS8XXX_REG_I2C_DEBUGGING_ENABLE:
 	case PS8XXX_REG_MUX_IN_HPD_ASSERTION:
 	case PS8XXX_REG_BIST_CONT_MODE_BYTE0:
@@ -399,11 +401,13 @@ static int ps8xxx_emul_read_byte_workhorse(const struct emul *emul, int reg,
 			*val = data->dci_cfg;
 			return 0;
 		}
+		__fallthrough;
 	case PS8XXX_EMUL_PORT_GPIO:
 		if (reg == PS8805_REG_GPIO_CONTROL) {
 			*val = data->gpio_ctrl;
 			return 0;
 		}
+		__fallthrough;
 	case PS8XXX_EMUL_PORT_INVAL:
 		LOG_ERR("Invalid I2C address");
 		return -EIO;
@@ -481,11 +485,13 @@ static int ps8xxx_emul_write_byte_workhorse(const struct emul *emul, int reg,
 			data->dci_cfg = val;
 			return 0;
 		}
+		__fallthrough;
 	case PS8XXX_EMUL_PORT_GPIO:
 		if (reg == PS8805_REG_GPIO_CONTROL) {
 			data->gpio_ctrl = val;
 			return 0;
 		}
+		__fallthrough;
 	case PS8XXX_EMUL_PORT_INVAL:
 		LOG_ERR("Invalid I2C address");
 		return -EIO;
diff --git a/zephyr/emul/tcpc/emul_tcpci.c b/zephyr/emul/tcpc/emul_tcpci.c
index e19f7a2726..aa4e508489 100644
--- a/zephyr/emul/tcpc/emul_tcpci.c
+++ b/zephyr/emul/tcpc/emul_tcpci.c
@@ -1211,7 +1211,7 @@ static int tcpci_emul_handle_transmit(const struct emul *emul)
 	case TCPCI_MSG_TX_HARD_RESET:
 		tcpci_emul_disable_pd_msg_delivery(emul);
 		tcpci_emul_reset_mask_regs(ctx);
-		/* fallthrough */
+		__fallthrough;
 	case TCPCI_MSG_CABLE_RESET:
 		/*
 		 * Cable and Hard reset are special and set success and fail
@@ -1259,13 +1259,13 @@ int tcpci_emul_handle_write(const struct emul *emul, int reg, int msg_len)
 				ctx->write_data &= ~TCPC_REG_ALERT_RX_STATUS;
 			}
 		}
-	/* fallthrough */
+		__fallthrough;
 	case TCPC_REG_FAULT_STATUS:
 	case TCPC_REG_ALERT_EXT:
 		/* Clear bits where TCPM set 1 */
 		get_reg(ctx, reg, &alert_val);
 		ctx->write_data = alert_val & (~ctx->write_data);
-	/* fallthrough */
+		__fallthrough;
 	case TCPC_REG_ALERT_MASK:
 	case TCPC_REG_POWER_STATUS_MASK:
 	case TCPC_REG_FAULT_STATUS_MASK:
@@ -1344,6 +1344,7 @@ int tcpci_emul_handle_write(const struct emul *emul, int reg, int msg_len)
 	switch (reg_bytes) {
 	case 2:
 		rsvd_mask = tcpci_emul_rsvd_mask[reg + 1];
+		__fallthrough;
 	case 1:
 		rsvd_mask <<= 8;
 		rsvd_mask |= tcpci_emul_rsvd_mask[reg];
diff --git a/zephyr/emul/tcpc/emul_tcpci_partner_common.c b/zephyr/emul/tcpc/emul_tcpci_partner_common.c
index 1785acda51..22a9ddb91e 100644
--- a/zephyr/emul/tcpc/emul_tcpci_partner_common.c
+++ b/zephyr/emul/tcpc/emul_tcpci_partner_common.c
@@ -1076,7 +1076,7 @@ tcpci_partner_common_sop_msg_handler(struct tcpci_partner_data *data,
 
 		tcpci_partner_common_clear_ams_ctrl_msg(data);
 
-		/* Fall through */
+		__fallthrough;
 	case PD_CTRL_ACCEPT:
 		if (data->wait_for_response) {
 			if (data->in_soft_reset) {