From b92d21bd3198b60e241d9fc43815d30ca6fd203b Mon Sep 17 00:00:00 2001
From: Jacob Dahl <37091262+dakejahl@users.noreply.github.com>
Date: Fri, 12 Dec 2025 09:31:33 -0900
Subject: [PATCH] serial: add txSpaceAvailable function (#26069)

* serial: add txSpaceAvailable function

* serial: txSpaceAvailable and bytesAvailable fixups
---
 platforms/common/Serial.cpp                   |  5 ++++
 .../include/px4_platform_common/Serial.hpp    |  1 +
 platforms/nuttx/src/px4/common/SerialImpl.cpp | 26 ++++++++++++++++++-
 .../src/px4/common/include/SerialImpl.hpp     |  1 +
 platforms/posix/include/SerialImpl.hpp        |  1 +
 platforms/posix/src/px4/common/SerialImpl.cpp | 21 ++++++++++++++-
 platforms/qurt/include/SerialImpl.hpp         |  1 +
 platforms/qurt/src/px4/SerialImpl.cpp         | 21 ++++++++++++++-
 8 files changed, 74 insertions(+), 3 deletions(-)

diff --git a/platforms/common/Serial.cpp b/platforms/common/Serial.cpp
index 748dcba9b2..a129fe2492 100644
--- a/platforms/common/Serial.cpp
+++ b/platforms/common/Serial.cpp
@@ -74,6 +74,11 @@ ssize_t Serial::bytesAvailable()
 	return _impl.bytesAvailable();
 }
 
+ssize_t Serial::txSpaceAvailable()
+{
+	return _impl.txSpaceAvailable();
+}
+
 ssize_t Serial::read(uint8_t *buffer, size_t buffer_size)
 {
 	return _impl.read(buffer, buffer_size);
diff --git a/platforms/common/include/px4_platform_common/Serial.hpp b/platforms/common/include/px4_platform_common/Serial.hpp
index 983834ccf6..bf14035a5e 100644
--- a/platforms/common/include/px4_platform_common/Serial.hpp
+++ b/platforms/common/include/px4_platform_common/Serial.hpp
@@ -62,6 +62,7 @@ public:
 	bool close();
 
 	ssize_t bytesAvailable();
+	ssize_t txSpaceAvailable();
 	ssize_t read(uint8_t *buffer, size_t buffer_size);
 	ssize_t readAtLeast(uint8_t *buffer, size_t buffer_size, size_t character_count = 1, uint32_t timeout_ms = 0);
 
diff --git a/platforms/nuttx/src/px4/common/SerialImpl.cpp b/platforms/nuttx/src/px4/common/SerialImpl.cpp
index f8968e4881..9bee2ddf23 100644
--- a/platforms/nuttx/src/px4/common/SerialImpl.cpp
+++ b/platforms/nuttx/src/px4/common/SerialImpl.cpp
@@ -293,12 +293,36 @@ ssize_t SerialImpl::bytesAvailable()
 {
 	if (!_open) {
 		PX4_ERR("Device not open!");
+		errno = EBADF;
 		return -1;
 	}
 
 	ssize_t bytes_available = 0;
 	int ret = ioctl(_serial_fd, FIONREAD, &bytes_available);
-	return ret >= 0 ? bytes_available : 0;
+
+	if (ret < 0) {
+		return -1;
+	}
+
+	return bytes_available;
+}
+
+ssize_t SerialImpl::txSpaceAvailable()
+{
+	if (!_open) {
+		PX4_ERR("Device not open!");
+		errno = EBADF;
+		return -1;
+	}
+
+	ssize_t space_available = 0;
+	int ret = ioctl(_serial_fd, FIONSPACE, &space_available);
+
+	if (ret < 0) {
+		return -1;
+	}
+
+	return space_available;
 }
 
 ssize_t SerialImpl::read(uint8_t *buffer, size_t buffer_size)
diff --git a/platforms/nuttx/src/px4/common/include/SerialImpl.hpp b/platforms/nuttx/src/px4/common/include/SerialImpl.hpp
index 82e4ddd29d..32c549ad6e 100644
--- a/platforms/nuttx/src/px4/common/include/SerialImpl.hpp
+++ b/platforms/nuttx/src/px4/common/include/SerialImpl.hpp
@@ -60,6 +60,7 @@ public:
 	bool close();
 
 	ssize_t bytesAvailable();
+	ssize_t txSpaceAvailable();
 	ssize_t read(uint8_t *buffer, size_t buffer_size);
 	ssize_t readAtLeast(uint8_t *buffer, size_t buffer_size, size_t character_count = 1, uint32_t timeout_us = 0);
 
diff --git a/platforms/posix/include/SerialImpl.hpp b/platforms/posix/include/SerialImpl.hpp
index 82e4ddd29d..32c549ad6e 100644
--- a/platforms/posix/include/SerialImpl.hpp
+++ b/platforms/posix/include/SerialImpl.hpp
@@ -60,6 +60,7 @@ public:
 	bool close();
 
 	ssize_t bytesAvailable();
+	ssize_t txSpaceAvailable();
 	ssize_t read(uint8_t *buffer, size_t buffer_size);
 	ssize_t readAtLeast(uint8_t *buffer, size_t buffer_size, size_t character_count = 1, uint32_t timeout_us = 0);
 
diff --git a/platforms/posix/src/px4/common/SerialImpl.cpp b/platforms/posix/src/px4/common/SerialImpl.cpp
index cb12b6919d..4859832829 100644
--- a/platforms/posix/src/px4/common/SerialImpl.cpp
+++ b/platforms/posix/src/px4/common/SerialImpl.cpp
@@ -281,12 +281,31 @@ ssize_t SerialImpl::bytesAvailable()
 {
 	if (!_open) {
 		PX4_ERR("Device not open!");
+		errno = EBADF;
 		return -1;
 	}
 
 	ssize_t bytes_available = 0;
 	int ret = ioctl(_serial_fd, FIONREAD, &bytes_available);
-	return ret >= 0 ? bytes_available : 0;
+
+	if (ret < 0) {
+		return -1;
+	}
+
+	return bytes_available;
+}
+
+ssize_t SerialImpl::txSpaceAvailable()
+{
+	if (!_open) {
+		PX4_ERR("Device not open!");
+		errno = EBADF;
+		return -1;
+	}
+
+	// POSIX/Linux doesn't have a direct equivalent to NuttX's FIONSPACE
+	errno = ENOSYS;
+	return -1;
 }
 
 ssize_t SerialImpl::read(uint8_t *buffer, size_t buffer_size)
diff --git a/platforms/qurt/include/SerialImpl.hpp b/platforms/qurt/include/SerialImpl.hpp
index c7739429ef..09797a33f7 100644
--- a/platforms/qurt/include/SerialImpl.hpp
+++ b/platforms/qurt/include/SerialImpl.hpp
@@ -59,6 +59,7 @@ public:
 	bool close();
 
 	ssize_t bytesAvailable();
+	ssize_t txSpaceAvailable();
 	ssize_t read(uint8_t *buffer, size_t buffer_size);
 	ssize_t readAtLeast(uint8_t *buffer, size_t buffer_size, size_t character_count = 1, uint32_t timeout_us = 0);
 
diff --git a/platforms/qurt/src/px4/SerialImpl.cpp b/platforms/qurt/src/px4/SerialImpl.cpp
index 5c31be9a5c..c7d78baffe 100644
--- a/platforms/qurt/src/px4/SerialImpl.cpp
+++ b/platforms/qurt/src/px4/SerialImpl.cpp
@@ -156,14 +156,33 @@ ssize_t SerialImpl::bytesAvailable()
 {
 	if (!_open) {
 		PX4_ERR("Device not open!");
+		errno = EBADF;
 		return -1;
 	}
 
 	uint32_t rx_bytes = 0;
-	(void) fc_uart_rx_available(_serial_fd, &rx_bytes);
+	int ret = fc_uart_rx_available(_serial_fd, &rx_bytes);
+
+	if (ret < 0) {
+		return -1;
+	}
+
 	return (ssize_t) rx_bytes;
 }
 
+ssize_t SerialImpl::txSpaceAvailable()
+{
+	if (!_open) {
+		PX4_ERR("Device not open!");
+		errno = EBADF;
+		return -1;
+	}
+
+	// QURT doesn't have a direct equivalent to NuttX's FIONSPACE
+	errno = ENOSYS;
+	return -1;
+}
+
 ssize_t SerialImpl::read(uint8_t *buffer, size_t buffer_size)
 {
 	if (!_open) {