lib: use USB zero-copy transfers if possible

Decreases CPU load especially for embedded machines.
Requires Linux >= 4.6 and libusb >= 1.0.21. If this
is not the case or the allocation fails, we will
fall back to buffers allocated in userspace.

Signed-off-by: Steve Markgraf <steve@steve-m.de>

src/librtlsdr.c

@@ -103,6 +103,7 @@ struct rtlsdr_dev {
 	void *cb_ctx;
 	enum rtlsdr_async_status async_status;
 	int async_cancel;
+	int use_zerocopy;
 	/* rtl demod context */
 	uint32_t rate; /* Hz */
 	uint32_t rtl_xtal; /* Hz */
@@ -1741,12 +1742,49 @@ static int _rtlsdr_alloc_async_buffers(rtlsdr_dev_t *dev)
 			dev->xfer[i] = libusb_alloc_transfer(0);
 	}
 
-	if (!dev->xfer_buf) {
-		dev->xfer_buf = malloc(dev->xfer_buf_num *
-					   sizeof(unsigned char *));
+	if (dev->xfer_buf)
+		return -2;
 
-		for(i = 0; i < dev->xfer_buf_num; ++i)
+	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
+	fprintf(stderr, "Allocating %d zero-copy buffers\n", dev->xfer_buf_num);
+
+	dev->use_zerocopy = 1;
+	for (i = 0; i < dev->xfer_buf_num; ++i) {
+		dev->xfer_buf[i] = libusb_dev_mem_alloc(dev->devh, dev->xfer_buf_len);
+
+		if (!dev->xfer_buf[i]) {
+			fprintf(stderr, "Failed to allocate zero-copy "
+					"buffer for transfer %d\nFalling "
+					"back to buffers in userspace\n", i);
+
+			dev->use_zerocopy = 0;
+			break;
+		}
+	}
+
+	/* zero-copy buffer allocation failed (partially or completely)
+	 * we need to free the buffers again if already allocated */
+	if (!dev->use_zerocopy) {
+		for (i = 0; i < dev->xfer_buf_num; ++i) {
+			if (dev->xfer_buf[i])
+				libusb_dev_mem_free(dev->devh,
+						    dev->xfer_buf[i],
+						    dev->xfer_buf_len);
+		}
+	}
+#endif
+
+	/* no zero-copy available, allocate buffers in userspace */
+	if (!dev->use_zerocopy) {
+		for (i = 0; i < dev->xfer_buf_num; ++i) {
 			dev->xfer_buf[i] = malloc(dev->xfer_buf_len);
+
+			if (!dev->xfer_buf[i])
+				return -ENOMEM;
+		}
 	}
 
 	return 0;
@@ -1772,9 +1810,18 @@ static int _rtlsdr_free_async_buffers(rtlsdr_dev_t *dev)
 
 	if (dev->xfer_buf) {
 		for (i = 0; i < dev->xfer_buf_num; ++i) {
-			if (dev->xfer_buf[i])
+			if (dev->xfer_buf[i]) {
+				if (dev->use_zerocopy) {
+#if defined (__linux__) && LIBUSB_API_VERSION >= 0x01000105
+					libusb_dev_mem_free(dev->devh,
+							    dev->xfer_buf[i],
+							    dev->xfer_buf_len);
+#endif
+				} else {
 					free(dev->xfer_buf[i]);
 				}
+			}
+		}
 
 		free(dev->xfer_buf);
 		dev->xfer_buf = NULL;
@@ -1828,7 +1875,12 @@ int rtlsdr_read_async(rtlsdr_dev_t *dev, rtlsdr_read_async_cb_t cb, void *ctx,
 
 		r = libusb_submit_transfer(dev->xfer[i]);
 		if (r < 0) {
-			fprintf(stderr, "Failed to submit transfer %i!\n", i);
+			fprintf(stderr, "Failed to submit transfer %i\n"
+					"Please increase your allowed " 
+					"usbfs buffer size with the "
+					"following command:\n"
+					"echo 0 > /sys/module/usbcore"
+					"/parameters/usbfs_memory_mb\n", i);
 			dev->async_status = RTLSDR_CANCELING;
 			break;
 		}