Use the termios v2 to custom baud rate switching

For a long time the Linux kernel has support new version of
the termios structure (V2). To setup the user speed it is
necessary to do following steps:

* Query current content of termios2 by calling the ioctl() with
  the TCGETS2 flag.

* Add to the c_cflag field the BOTHER flag.

* Set to the c_ispeed/c_ospeed fields an actual values of custom speeds.

* Write new content of termios2 back by calling the ioctl() with the
  TCSETS2 flag.

This new method much simpler and is more transparent in implementation
than changes of a custom divisor. It is preferable and will be used
by default for all cases. If for some reason a current Linux kernel
doesn't support the termios2, then will be falling back to the old
method with changing of a custom divisor.

Tested with the on-board and the USB (PL2303) serial ports.

(cherry-picked from eb0535c9701202d5f74fe1fc9b3b094bbecb8043)
Task-number: QTBUG-48094
Change-Id: I49a5389b089980b616b4e2ff815ce0b579752d0e
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>

2 files changed, 66 insertions, 1 deletions

diff --git a/src/serialport/qserialport_unix.cpp b/src/serialport/qserialport_unix.cpp
index b6194d08..7954cc3d 100644
--- a/src/serialport/qserialport_unix.cpp
+++ b/src/serialport/qserialport_unix.cpp
@@ -60,6 +60,33 @@
 #define CRTSCTS (IHFLOW | OHFLOW)
 #endif
 
+#ifdef Q_OS_LINUX
+
+struct termios2 {
+    tcflag_t c_iflag;       /* input mode flags */
+    tcflag_t c_oflag;       /* output mode flags */
+    tcflag_t c_cflag;       /* control mode flags */
+    tcflag_t c_lflag;       /* local mode flags */
+    cc_t c_line;            /* line discipline */
+    cc_t c_cc[19];          /* control characters */
+    speed_t c_ispeed;       /* input speed */
+    speed_t c_ospeed;       /* output speed */
+};
+
+#ifndef TCGETS2
+#define TCGETS2     _IOR('T', 0x2A, struct termios2)
+#endif
+
+#ifndef TCSETS2
+#define TCSETS2     _IOW('T', 0x2B, struct termios2)
+#endif
+
+#ifndef BOTHER
+#define BOTHER      0010000
+#endif
+
+#endif
+
 #include <private/qcore_unix_p.h>
 
 #include <QtCore/qelapsedtimer.h>
@@ -430,7 +457,17 @@ bool QSerialPortPrivate::setBaudRate()
 bool QSerialPortPrivate::setStandardBaudRate(qint32 baudRate, QSerialPort::Directions directions)
 {
 #ifdef Q_OS_LINUX
-    // try to clear custom baud rate
+    // try to clear custom baud rate, using termios v2
+    struct termios2 tio2;
+    if (::ioctl(descriptor, TCGETS2, &tio2) != -1) {
+        if (tio2.c_cflag & BOTHER) {
+            tio2.c_cflag &= ~BOTHER;
+            tio2.c_cflag |= CBAUD;
+            ::ioctl(descriptor, TCSETS2, &tio2);
+        }
+    }
+
+    // try to clear custom baud rate, using serial_struct (old way)
     struct serial_struct serial;
     ::memset(&serial, 0, sizeof(serial));
     if (::ioctl(descriptor, TIOCGSERIAL, &serial) != -1) {
@@ -464,6 +501,24 @@ bool QSerialPortPrivate::setStandardBaudRate(qint32 baudRate, QSerialPort::Direc
 
 bool QSerialPortPrivate::setCustomBaudRate(qint32 baudRate, QSerialPort::Directions directions)
 {
+    struct termios2 tio2;
+
+    if (::ioctl(descriptor, TCGETS2, &tio2) != -1) {
+        tio2.c_cflag &= ~CBAUD;
+        tio2.c_cflag |= BOTHER;
+
+        if (directions & QSerialPort::Input)
+            tio2.c_ispeed = baudRate;
+
+        if (directions & QSerialPort::Output)
+            tio2.c_ospeed = baudRate;
+
+        if (::ioctl(descriptor, TCSETS2, &tio2) != -1
+                && ::ioctl(descriptor, TCGETS2, &tio2) != -1) {
+            return true;
+        }
+    }
+
     struct serial_struct serial;
 
     if (::ioctl(descriptor, TIOCGSERIAL, &serial) == -1) {
diff --git a/tests/auto/qserialport/tst_qserialport.cpp b/tests/auto/qserialport/tst_qserialport.cpp
index cc0fb3c4..8aeca3c1 100644
--- a/tests/auto/qserialport/tst_qserialport.cpp
+++ b/tests/auto/qserialport/tst_qserialport.cpp
@@ -344,6 +344,8 @@ void tst_QSerialPort::baudRate_data()
     QTest::newRow("Baud38400") << static_cast<qint32>(QSerialPort::Baud38400);
     QTest::newRow("Baud57600") << static_cast<qint32>(QSerialPort::Baud57600);
     QTest::newRow("Baud115200") << static_cast<qint32>(QSerialPort::Baud115200);
+
+    QTest::newRow("31250") << 31250; // custom baudrate (MIDI)
 }
 
 void tst_QSerialPort::baudRate()
@@ -1098,6 +1100,14 @@ void tst_QSerialPort::readWriteWithDifferentBaudRate_data()
     QTest::newRow("9600, 9600") << 9600 << 9600 << true;
     QTest::newRow("115200, 115200") << 115200 << 115200 << true;
     QTest::newRow("9600, 115200") << 9600 << 115200 << false;
+
+    QTest::newRow("31250, 31250") << 31250 << 31250 << true; // custom baudrate (MIDI)
+    QTest::newRow("31250, 115200") << 31250 << 115200 << false;
+
+#ifdef Q_OS_LINUX
+    QTest::newRow("14400, 14400") << 14400 << 14400 << true; // custom baudrate for Linux
+    QTest::newRow("14400, 115200") << 14400 << 115200 << false;
+#endif
 }
 
 void tst_QSerialPort::readWriteWithDifferentBaudRate()