Increase buffer chunk sizes

This reduces the number of syscalls and thus the CPU load when dealing
with fast ports. And 32KiB seems still small enough to be no concern on
modern hardware even if it's excessive for a given port.

Change-Id: I4749e4255627d3abf2393323216e01ac6436413b
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/serialport/qserialport_win.cpp b/src/serialport/qserialport_win.cpp
index 43660ff7..67c68ee8 100644
--- a/src/serialport/qserialport_win.cpp
+++ b/src/serialport/qserialport_win.cpp
@@ -355,7 +355,7 @@ bool QSerialPortPrivate::waitForReadyRead(int msecs)
 
         if (overlapped == &readCompletionOverlapped) {
             const qint64 readBytesForOneReadOperation = qint64(buffer.size()) - currentReadBufferSize;
-            if (readBytesForOneReadOperation == ReadChunkSize) {
+            if (readBytesForOneReadOperation == QSERIALPORT_BUFFERSIZE) {
                 currentReadBufferSize = buffer.size();
             } else if (readBytesForOneReadOperation == 0) {
                 if (initialReadBufferSize != currentReadBufferSize)
@@ -481,7 +481,7 @@ bool QSerialPortPrivate::completeAsyncRead(qint64 bytesTransferred)
     readStarted = false;
 
     bool result = true;
-    if (bytesTransferred == ReadChunkSize
+    if (bytesTransferred == QSERIALPORT_BUFFERSIZE
             || queuedBytesCount(QSerialPort::Input) > 0) {
         result = startAsyncRead();
     } else if (readBufferMaxSize == 0
@@ -538,7 +538,7 @@ bool QSerialPortPrivate::startAsyncRead()
     if (readStarted)
         return true;
 
-    DWORD bytesToRead = ReadChunkSize;
+    DWORD bytesToRead = QSERIALPORT_BUFFERSIZE;
 
     if (readBufferMaxSize && bytesToRead > (readBufferMaxSize - buffer.size())) {
         bytesToRead = readBufferMaxSize - buffer.size();
@@ -549,6 +549,8 @@ bool QSerialPortPrivate::startAsyncRead()
         }
     }
 
+    Q_ASSERT(int(bytesToRead) <= readChunkBuffer.size());
+
     ::ZeroMemory(&readCompletionOverlapped, sizeof(readCompletionOverlapped));
     if (::ReadFile(handle, readChunkBuffer.data(), bytesToRead, nullptr, &readCompletionOverlapped)) {
         readStarted = true;