output modules simplify locking for mixer reset and load

- fixes the deadlock in px4io ioctl mixer reset
   - px4io Run() locks (CDev semaphore)
   - mixer load goes through px4io ioctl MIXERIOCRESET which calls MixingOutput::resetMixerThreadSafe()
   - MixingOutput::resetMixerThreadSafe() stores a Command::Type::resetMixer command in an atomic variable, schedules a work queue cycle, then sleep spins until the command is cleared
   - the execution of the cycle eventually calls back into PX4IO::updateOutputs(), which tries to lock (and waits forever)

src/drivers/dshot/DShot.cpp

@@ -1,6 +1,6 @@
 /****************************************************************************
  *
- *   Copyright (c) 2019-2021 PX4 Development Team. All rights reserved.
+ *   Copyright (c) 2019-2022 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -35,6 +35,8 @@
 
 #include <px4_arch/io_timer.h>
 
+#include <px4_platform_common/sem.hpp>
+
 char DShot::_telemetry_device[] {};
 px4::atomic_bool DShot::_request_telemetry_init{false};
 
@@ -487,6 +489,8 @@ bool DShot::updateOutputs(bool stop_motors, uint16_t outputs[MAX_ACTUATORS],
 
 void DShot::Run()
 {
+	SmartLock lock_guard(_lock);
+
 	if (should_exit()) {
 		ScheduleClear();
 		_mixing_output.unregister();
@@ -640,21 +644,21 @@ void DShot::update_params()
 
 int DShot::ioctl(file *filp, int cmd, unsigned long arg)
 {
+	SmartLock lock_guard(_lock);
+
 	int ret = OK;
 
 	PX4_DEBUG("dshot ioctl cmd: %d, arg: %ld", cmd, arg);
 
-	lock();
-
 	switch (cmd) {
 	case MIXERIOCRESET:
-		_mixing_output.resetMixerThreadSafe();
+		_mixing_output.resetMixer();
 		break;
 
 	case MIXERIOCLOADBUF: {
 			const char *buf = (const char *)arg;
 			unsigned buflen = strlen(buf);
-			ret = _mixing_output.loadMixerThreadSafe(buf, buflen);
+			ret = _mixing_output.loadMixer(buf, buflen);
 
 			break;
 		}
@@ -664,8 +668,6 @@ int DShot::ioctl(file *filp, int cmd, unsigned long arg)
 		break;
 	}
 
-	unlock();
-
 	// if nobody wants it, let CDev have it
 	if (ret == -ENOTTY) {
 		ret = CDev::ioctl(filp, cmd, arg);