From d3bc369164494438627c9ac82c6ca9421bf08644 Mon Sep 17 00:00:00 2001
From: Hermes <piotr@piksar.eu>
Date: Thu, 1 Oct 2026 15:26:24 +0200
Subject: [PATCH] fix: set 115200 baud via CBAUD bits and verify readback on
 UART open (#10)

---
 internal/serialport/serial_linux.go      | 44 ++++++++++++++----
 internal/serialport/serial_linux_test.go | 59 ++++++++++++++++++++++++
 2 files changed, 94 insertions(+), 9 deletions(-)
 create mode 100644 internal/serialport/serial_linux_test.go

diff --git a/internal/serialport/serial_linux.go b/internal/serialport/serial_linux.go
index ec6472c..3e407a2 100644
--- a/internal/serialport/serial_linux.go
+++ b/internal/serialport/serial_linux.go
@@ -19,19 +19,45 @@ func Open115200(path string) (*os.File, error) {
 		return nil, err
 	}
 
-	termios.Iflag = 0
-	termios.Oflag = 0
-	termios.Lflag = 0
-	termios.Cflag &^= unix.CSIZE | unix.PARENB | unix.CSTOPB | unix.CRTSCTS
-	termios.Cflag |= unix.CS8 | unix.CREAD | unix.CLOCAL
-	termios.Ispeed = unix.B115200
-	termios.Ospeed = unix.B115200
-	termios.Cc[unix.VMIN] = 1
-	termios.Cc[unix.VTIME] = 0
+	configureRaw115200(termios)
 
 	if err := unix.IoctlSetTermios(int(file.Fd()), unix.TCSETS, termios); err != nil {
 		file.Close()
 		return nil, fmt.Errorf("configure %s: %w", path, err)
 	}
+
+	effective, err := unix.IoctlGetTermios(int(file.Fd()), unix.TCGETS)
+	if err != nil {
+		file.Close()
+		return nil, fmt.Errorf("read back %s: %w", path, err)
+	}
+	if err := verifyBaud(*effective, unix.B115200); err != nil {
+		file.Close()
+		return nil, fmt.Errorf("verify %s: %w", path, err)
+	}
 	return file, nil
 }
+
+// configureRaw115200 keeps the raw 115200 8N1 blocking-read configuration.
+// On Linux TCSETS takes the line speed from the CBAUD bits in Cflag, so
+// CBAUD must be cleared and B115200 selected there; Ispeed/Ospeed alone
+// leave an inherited baud (for example 9600 from boot defaults) in effect.
+func configureRaw115200(termios *unix.Termios) {
+	termios.Iflag = 0
+	termios.Oflag = 0
+	termios.Lflag = 0
+	termios.Cflag &^= unix.CSIZE | unix.PARENB | unix.CSTOPB | unix.CRTSCTS | unix.CBAUD
+	termios.Cflag |= unix.B115200
+	termios.Cflag |= unix.CS8 | unix.CREAD | unix.CLOCAL
+	termios.Ispeed = unix.B115200
+	termios.Ospeed = unix.B115200
+	termios.Cc[unix.VMIN] = 1
+	termios.Cc[unix.VTIME] = 0
+}
+
+func verifyBaud(termios unix.Termios, baud uint32) error {
+	if got := termios.Cflag & unix.CBAUD; got != baud {
+		return fmt.Errorf("effective baud bits %#o, want %#o", got, baud)
+	}
+	return nil
+}
diff --git a/internal/serialport/serial_linux_test.go b/internal/serialport/serial_linux_test.go
new file mode 100644
index 0000000..314bcfb
--- /dev/null
+++ b/internal/serialport/serial_linux_test.go
@@ -0,0 +1,59 @@
+package serialport
+
+import (
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestConfigureRaw115200OverridesInheritedBaud(t *testing.T) {
+	var termios unix.Termios
+	termios.Iflag = unix.IGNBRK | unix.ICRNL
+	termios.Oflag = unix.OPOST
+	termios.Lflag = unix.ICANON | unix.ECHO
+	termios.Cflag = unix.B9600 | unix.CS8 | unix.CREAD | unix.CLOCAL | unix.HUPCL
+	termios.Ispeed = unix.B9600
+	termios.Ospeed = unix.B9600
+
+	configureRaw115200(&termios)
+
+	if got := termios.Cflag & unix.CBAUD; got != unix.B115200 {
+		t.Fatalf("effective baud bits=%#o want %#o", got, unix.B115200)
+	}
+	if got := termios.Cflag & unix.CSIZE; got != unix.CS8 {
+		t.Fatalf("character size bits=%#o want %#o", got, unix.CS8)
+	}
+	for _, flag := range []uint32{unix.CREAD, unix.CLOCAL} {
+		if termios.Cflag&flag == 0 {
+			t.Fatalf("flag %#o lost", flag)
+		}
+	}
+	for _, flag := range []uint32{unix.PARENB, unix.CSTOPB, unix.CRTSCTS} {
+		if termios.Cflag&flag != 0 {
+			t.Fatalf("flag %#o must be cleared", flag)
+		}
+	}
+	if termios.Iflag != 0 || termios.Oflag != 0 || termios.Lflag != 0 {
+		t.Fatal("raw mode must clear Iflag, Oflag and Lflag")
+	}
+	if termios.Ispeed != unix.B115200 || termios.Ospeed != unix.B115200 {
+		t.Fatalf("speed fields=%d/%d want 115200/115200", termios.Ispeed, termios.Ospeed)
+	}
+	if termios.Cc[unix.VMIN] != 1 || termios.Cc[unix.VTIME] != 0 {
+		t.Fatalf("VMIN=%d VTIME=%d want 1/0", termios.Cc[unix.VMIN], termios.Cc[unix.VTIME])
+	}
+}
+
+func TestVerifyBaudAcceptsOnlyConfiguredLineSpeed(t *testing.T) {
+	var inherited unix.Termios
+	inherited.Cflag = unix.B9600 | unix.CS8
+	if err := verifyBaud(inherited, unix.B115200); err == nil {
+		t.Fatal("inherited 9600 line speed must fail verification")
+	}
+
+	var configured unix.Termios
+	configured.Cflag = unix.B115200 | unix.CS8 | unix.CREAD
+	if err := verifyBaud(configured, unix.B115200); err != nil {
+		t.Fatalf("115200 line speed must verify: %v", err)
+	}
+}
-- 
2.54.0