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>

4 files changed, 19 insertions, 12 deletions

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) {