From 143b51852837505ea7748a4efbcf0b8d69ce98b7 Mon Sep 17 00:00:00 2001
From: Gennaro Guidone <99129829+gguidone@users.noreply.github.com>
Date: Wed, 17 Jun 2026 16:37:50 +0200
Subject: [PATCH] fix(rcS): FMU output pins not working when serial passthrough
 is not compiled for a board (#27682)

#27605 gated the dshot/pwm_out start on `param compare -s PASSTHRU_EN 0`.
PASSTHRU_EN is registered only when the serialpassthrough driver is
compiled in, which is not the case on most boards (e.g. fmu-v5x). There
param_find() fails, `param compare` returns "no match", the else branch
runs, and the FMU outputs never start: the PWM_AUX group disappears from
the Actuators screen and motors wired to those pins won't arm. SITL uses a
separate init.d-posix rcS and is unaffected, so CI did not catch it.

Use the start-unless-explicitly-enabled idiom already used elsewhere in
rcS (`param greater -s`): only skip the outputs when PASSTHRU_EN is
explicitly > 0. A missing or zero parameter starts the outputs as before.
---
 ROMFS/px4fmu_common/init.d/rcS | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/ROMFS/px4fmu_common/init.d/rcS b/ROMFS/px4fmu_common/init.d/rcS
index 93b5bfe2e70..92a7c3a89e7 100644
--- a/ROMFS/px4fmu_common/init.d/rcS
+++ b/ROMFS/px4fmu_common/init.d/rcS
@@ -539,12 +539,14 @@ else
 	else
 		commander start
 
-		if param compare -s PASSTHRU_EN 0
+		if param greater -s PASSTHRU_EN 0
 		then
+			# serial passthrough requested: leave the ESC lines free and
+			# reset so the next boot starts the outputs normally
+			param set PASSTHRU_EN 0
+		else
 			dshot start
 			pwm_out start
-		else
-			param set PASSTHRU_EN 0
 		fi
 	fi