lib: disable usbfs zero-copy support by default

Although we added a detection mechanism for the presence of the Kernel
bug earlier, reading from the incorrectly mapped memory might cause a
bus error on some ARM systems.

With the overall performance benefit being rather minimal for the
data rates of rtl-sdr, disable zero-copy by default.

CMakeLists.txt

@@ -126,6 +126,14 @@ else (DETACH_KERNEL_DRIVER)
     message (STATUS "Building with kernel driver detaching disabled, use -DDETACH_KERNEL_DRIVER=ON to enable")
 endif (DETACH_KERNEL_DRIVER)
 
+option(ENABLE_ZEROCOPY "Enable usbfs zero-copy support" OFF)
+if (ENABLE_ZEROCOPY)
+    message (STATUS "Building with usbfs zero-copy support enabled")
+    add_definitions(-DENABLE_ZEROCOPY=1)
+else (ENABLE_ZEROCOPY)
+    message (STATUS "Building with usbfs zero-copy support disabled, use -DENABLE_ZEROCOPY=ON to enable")
+endif (ENABLE_ZEROCOPY)
+
 ########################################################################
 # Add subdirectories
 ########################################################################

configure.ac

@@ -101,6 +101,12 @@ AC_ARG_ENABLE(driver-detach,
     CFLAGS="$CFLAGS -DDETACH_KERNEL_DRIVER"
 fi])
 
+AC_ARG_ENABLE(zerocopy,
+[  --enable-zerocopy          Enable usbfs zero-copy support (disabled by default)],
+[if test x$enableval = xyes; then
+    CFLAGS="$CFLAGS -DENABLE_ZEROCOPY"
+fi])
+
 dnl Generate the output
 AC_CONFIG_HEADER(config.h)
 

src/librtlsdr.c

@@ -1751,7 +1751,7 @@ static int _rtlsdr_alloc_async_buffers(rtlsdr_dev_t *dev)
 	dev->xfer_buf = malloc(dev->xfer_buf_num * sizeof(unsigned char *));
 	memset(dev->xfer_buf, 0, dev->xfer_buf_num * sizeof(unsigned char *));
 
-#if defined (__linux__) && LIBUSB_API_VERSION >= 0x01000105
+#if defined(ENABLE_ZEROCOPY) && defined (__linux__) && LIBUSB_API_VERSION >= 0x01000105
 	fprintf(stderr, "Allocating %d zero-copy buffers\n", dev->xfer_buf_num);
 
 	dev->use_zerocopy = 1;