[ncm] Use large multi-packet buffers by default

Some devices have a very small number of internal buffers, and rely on
being able to pack multiple packets into each buffer.  Using 2048-byte
buffers on such devices produces throughput of around 100Mbps.  Using
a small number of much larger buffers (e.g. 32kB) increases the
throughput to around 780Mbps.  (The full 1Gbps is not reached because
the high RTT induced by the use of multi-packet buffers causes us to
saturate our 256kB TCP window.)

Since allocation of large buffers is very likely to fail, allocate the
buffer set only once when the device is opened and recycle buffers
immediately after use.  Received data is now always copied to
per-packet buffers.

If allocation of large buffers fails, fall back to allocating a larger
number of smaller buffers.  This will give reduced performance, but
the device will at least still be functional.

Share code between the interrupt and bulk IN endpoint handlers, since
the buffer handling is now very similar.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/ncm.c

  *
  */
 
+/** Ring refill profiler */
+static struct profiler ncm_refill_profiler __profiler =
+	{ .name = "ncm.refill" };
+
 /** Interrupt completion profiler */
 static struct profiler ncm_intr_profiler __profiler =
 	{ .name = "ncm.intr" };
 
-/** Interrupt refill profiler */
-static struct profiler ncm_intr_refill_profiler __profiler =
-	{ .name = "ncm.intr_refill" };
-
 /** Bulk IN completion profiler */
 static struct profiler ncm_in_profiler __profiler =
 	{ .name = "ncm.in" };
 static struct profiler ncm_in_datagram_profiler __profiler =
 	{ .name = "ncm.in_dgram" };
 
-/** Bulk IN refill profiler */
-static struct profiler ncm_in_refill_profiler __profiler =
-	{ .name = "ncm.in_refill" };
-
 /** Bulk OUT profiler */
 static struct profiler ncm_out_profiler __profiler =
 	{ .name = "ncm.out" };
 
 /******************************************************************************
  *
- * CDC-NCM communications interface
+ * Ring management
  *
  ******************************************************************************
  */
 
 /**
- * Refill interrupt ring
+ * Transcribe receive ring name (for debugging)
+ *
+ * @v ncm		CDC-NCM device
+ * @v ring		Receive ring
+ * @ret name		Receive ring name
+ */
+static inline const char * ncm_rx_name ( struct ncm_device *ncm,
+					 struct ncm_rx_ring *ring ) {
+	if ( ring == &ncm->intr ) {
+		return "interrupt";
+	} else if ( ring == &ncm->in ) {
+		return "bulk IN";
+	} else {
+		return "UNKNOWN";
+	}
+}
+
+/**
+ * Allocate receive ring buffers
+ *
+ * @v ncm		CDC-NCM device
+ * @v ring		Receive ring
+ * @v mtu		I/O buffer size
+ * @v count		Number of I/O buffers
+ * @ret rc		Return status code
+ */
+static int ncm_rx_alloc ( struct ncm_device *ncm, struct ncm_rx_ring *ring,
+			  size_t mtu, unsigned int count ) {
+	struct io_buffer *iobuf;
+	struct io_buffer *tmp;
+	unsigned int i;
+	int rc;
+
+	/* Initialise ring */
+	ring->mtu = mtu;
+	INIT_LIST_HEAD ( &ring->list );
+
+	/* Allocate I/O buffers */
+	for ( i = 0 ; i < count ; i++ ) {
+		iobuf = alloc_iob ( mtu );
+		if ( ! iobuf ) {
+			DBGC ( ncm, "NCM %p could not allocate %dx %zd-byte "
+			       "buffers for %s\n", ncm, count, mtu,
+			       ncm_rx_name ( ncm, ring ) );
+			rc = -ENOMEM;
+			goto err_alloc;
+		}
+		list_add ( &iobuf->list, &ring->list );
+	}
+
+	return 0;
+
+ err_alloc:
+	list_for_each_entry_safe ( iobuf, tmp, &ring->list, list ) {
+		list_del ( &iobuf->list );
+		free_iob ( iobuf );
+	}
+	return rc;
+}
+
+/**
+ * Refill receive ring
  *
  * @v ncm		CDC-NCM device
+ * @v ring		Receive ring
+ * @ret rc		Return status code
  */
-static void ncm_intr_refill ( struct ncm_device *ncm ) {
+static int ncm_rx_refill ( struct ncm_device *ncm, struct ncm_rx_ring *ring ) {
 	struct io_buffer *iobuf;
-	size_t mtu = ncm->intr.mtu;
 	int rc;
 
-	/* Enqueue any available I/O buffers */
-	while ( ( iobuf = list_first_entry ( &ncm->intrs, struct io_buffer,
+	/* Enqueue any recycled I/O buffers */
+	while ( ( iobuf = list_first_entry ( &ring->list, struct io_buffer,
 					     list ) ) ) {
 
 		/* Profile refill */
-		profile_start ( &ncm_intr_refill_profiler );
+		profile_start ( &ncm_refill_profiler );
 
 		/* Reset size */
-		iob_put ( iobuf, ( mtu - iob_len ( iobuf ) ) );
+		iob_put ( iobuf, ( ring->mtu - iob_len ( iobuf ) ) );
 
 		/* Enqueue I/O buffer */
-		if ( ( rc = usb_stream ( &ncm->intr, iobuf ) ) != 0 ) {
-			DBGC ( ncm, "NCM %p could not enqueue interrupt: %s\n",
-			       ncm, strerror ( rc ) );
-			/* Leave in available list and wait for next refill */
-			return;
+		if ( ( rc = usb_stream ( &ring->ep, iobuf ) ) != 0 ) {
+			DBGC ( ncm, "NCM %p could not enqueue %s: %s\n", ncm,
+			       ncm_rx_name ( ncm, ring ), strerror ( rc ) );
+			/* Leave in recycled list and wait for next refill */
+			return rc;
 		}
 
-		/* Remove from available list */
+		/* Remove from recycled list */
 		list_del ( &iobuf->list );
-		profile_stop ( &ncm_intr_refill_profiler );
+		profile_stop ( &ncm_refill_profiler );
 	}
+
+	return 0;
+}
+
+/**
+ * Recycle receive buffer
+ *
+ * @v ncm		CDC-NCM device
+ * @v ring		Receive ring
+ * @v iobuf		I/O buffer
+ */
+static inline void ncm_rx_recycle ( struct ncm_device *ncm __unused,
+				    struct ncm_rx_ring *ring,
+				    struct io_buffer *iobuf ) {
+
+	/* Add to recycled list */
+	list_add_tail ( &iobuf->list, &ring->list );
 }
 
+/**
+ * Free receive ring
+ *
+ * @v ncm		CDC-NCM device
+ * @v ring		Receive ring
+ */
+static void ncm_rx_free ( struct ncm_device *ncm __unused,
+			  struct ncm_rx_ring *ring ) {
+	struct io_buffer *iobuf;
+	struct io_buffer *tmp;
+
+	/* Free I/O buffers */
+	list_for_each_entry_safe ( iobuf, tmp, &ring->list, list ) {
+		list_del ( &iobuf->list );
+		free_iob ( iobuf );
+	}
+}
+
+/******************************************************************************
+ *
+ * CDC-NCM communications interface
+ *
+ ******************************************************************************
+ */
+
 /**
  * Complete interrupt transfer
  *
  */
 static void ncm_intr_complete ( struct usb_endpoint *ep,
 				struct io_buffer *iobuf, int rc ) {
-	struct ncm_device *ncm = container_of ( ep, struct ncm_device, intr );
+	struct ncm_device *ncm = container_of ( ep, struct ncm_device, intr.ep);
 	struct net_device *netdev = ncm->netdev;
 	struct usb_setup_packet *message;
 	size_t len = iob_len ( iobuf );
 	}
 
  done:
-	/* Return I/O buffer to available list */
-	list_add_tail ( &iobuf->list, &ncm->intrs );
+	/* Recycle buffer */
+	ncm_rx_recycle ( ncm, &ncm->intr, iobuf );
 	profile_stop ( &ncm_intr_profiler );
 }
 
  * @ret rc		Return status code
  */
 static int ncm_comms_open ( struct ncm_device *ncm ) {
-	struct io_buffer *iobuf;
-	struct io_buffer *tmp;
-	unsigned int i;
 	int rc;
 
 	/* Allocate I/O buffers */
-	for ( i = 0 ; i < NCM_INTR_FILL ; i++ ) {
-		iobuf = alloc_iob ( ncm->intr.mtu );
-		if ( ! iobuf ) {
-			rc = -ENOMEM;
-			goto err_alloc_iob;
-		}
-		list_add ( &iobuf->list, &ncm->intrs );
+	if ( ( rc = ncm_rx_alloc ( ncm, &ncm->intr, ncm->intr.ep.mtu,
+				   NCM_INTR_COUNT ) ) != 0 ) {
+		DBGC ( ncm, "NCM %p could not allocate RX buffers: %s\n",
+		       ncm, strerror ( rc ) );
+		goto err_alloc;
 	}
 
 	/* Open interrupt endpoint */
-	if ( ( rc = usb_endpoint_open ( &ncm->intr ) ) != 0 ) {
+	if ( ( rc = usb_endpoint_open ( &ncm->intr.ep ) ) != 0 ) {
 		DBGC ( ncm, "NCM %p could not open interrupt: %s\n",
 		       ncm, strerror ( rc ) );
 		goto err_open;
 
 	return 0;
 
-	usb_endpoint_close ( &ncm->intr );
+	usb_endpoint_close ( &ncm->intr.ep );
  err_open:
- err_alloc_iob:
-	list_for_each_entry_safe ( iobuf, tmp, &ncm->intrs, list ) {
-		list_del ( &iobuf->list );
-		free_iob ( iobuf );
-	}
+	ncm_rx_free ( ncm, &ncm->intr );
+ err_alloc:
 	return rc;
 }
 
  * @v ncm		CDC-NCM device
  */
 static void ncm_comms_close ( struct ncm_device *ncm ) {
-	struct io_buffer *iobuf;
-	struct io_buffer *tmp;
 
 	/* Close interrupt endpoint */
-	usb_endpoint_close ( &ncm->intr );
+	usb_endpoint_close ( &ncm->intr.ep );
 
 	/* Free I/O buffers */
-	list_for_each_entry_safe ( iobuf, tmp, &ncm->intrs, list ) {
-		list_del ( &iobuf->list );
-		free_iob ( iobuf );
-	}
+	ncm_rx_free ( ncm, &ncm->intr );
 }
 
 /******************************************************************************
  */
 
 /**
- * Refill bulk IN ring
+ * Allocate bulk IN receive ring buffers
  *
  * @v ncm		CDC-NCM device
+ * @ret rc		Return status code
  */
-static void ncm_in_refill ( struct ncm_device *ncm ) {
-	struct net_device *netdev = ncm->netdev;
-	struct io_buffer *iobuf;
+static int ncm_in_alloc ( struct ncm_device *ncm ) {
+	size_t mtu;
+	unsigned int count;
 	int rc;
 
-	/* Refill ring */
-	while ( ncm->fill < NCM_IN_FILL ) {
-
-		/* Profile refill */
-		profile_start ( &ncm_in_refill_profiler );
-
-		/* Allocate I/O buffer */
-		iobuf = alloc_iob ( NCM_NTB_INPUT_SIZE );
-		if ( ! iobuf ) {
-			/* Wait for next refill */
-			break;
-		}
-		iob_put ( iobuf, NCM_NTB_INPUT_SIZE );
-
-		/* Enqueue I/O buffer */
-		if ( ( rc = usb_stream ( &ncm->in, iobuf ) ) != 0 ) {
-			netdev_rx_err ( netdev, iobuf, rc );
-			break;
-		}
-
-		/* Increment fill level */
-		ncm->fill++;
-		profile_stop ( &ncm_in_refill_profiler );
+	/* Some devices have a very small number of internal buffers,
+	 * and rely on being able to pack multiple packets into each
+	 * buffer.  We therefore want to use large buffers if
+	 * possible.  However, large allocations have a reasonable
+	 * chance of failure, especially if this is not the first or
+	 * only device to be opened.
+	 *
+	 * We therefore attempt to find a usable buffer size, starting
+	 * large and working downwards until allocation succeeds.
+	 * Smaller buffers will still work, albeit with a higher
+	 * chance of packet loss and so lower overall throughput.
+	 */
+	for ( mtu = ncm->mtu ; mtu >= NCM_MIN_NTB_INPUT_SIZE ; mtu >>= 1 ) {
+
+		/* Attempt allocation at this MTU */
+		if ( mtu > NCM_MAX_NTB_INPUT_SIZE )
+			continue;
+		count = ( NCM_IN_MIN_SIZE / mtu );
+		if ( count < NCM_IN_MIN_COUNT )
+			count = NCM_IN_MIN_COUNT;
+		if ( ( count * mtu ) > NCM_IN_MAX_SIZE )
+			continue;
+		if ( ( rc = ncm_rx_alloc ( ncm, &ncm->in, mtu, count ) ) != 0 )
+			continue;
+
+		DBGC ( ncm, "NCM %p using %dx %zd-byte buffers for bulk IN\n",
+		       ncm, count, mtu );
+		return 0;
 	}
+
+	DBGC ( ncm, "NCM %p could not allocate bulk IN buffers\n", ncm );
+	return -ENOMEM;
 }
 
 /**
  */
 static void ncm_in_complete ( struct usb_endpoint *ep, struct io_buffer *iobuf,
 			      int rc ) {
-	struct ncm_device *ncm = container_of ( ep, struct ncm_device, in );
+	struct ncm_device *ncm = container_of ( ep, struct ncm_device, in.ep );
 	struct net_device *netdev = ncm->netdev;
 	struct ncm_transfer_header *nth;
 	struct ncm_datagram_pointer *ndp;
 	/* Profile overall bulk IN completion */
 	profile_start ( &ncm_in_profiler );
 
-	/* Decrement fill level */
-	ncm->fill--;
-
 	/* Ignore packets cancelled when the endpoint closes */
 	if ( ! ep->open )
 		goto ignore;
 
 	/* Record USB errors against the network device */
-	if ( rc != 0 )
+	if ( rc != 0 ) {
+		DBGC ( ncm, "NCM %p bulk IN failed: %s\n",
+		       ncm, strerror ( rc ) );
 		goto drop;
+	}
 
 	/* Locate transfer header */
 	len = iob_len ( iobuf );
 		/* Move to next descriptor */
 		desc++;
 
-		/* Create new I/O buffer if necessary */
-		if ( remaining && desc->offset ) {
-			/* More packets remain: create new buffer */
-			pkt = alloc_iob ( pkt_len );
-			if ( ! pkt ) {
-				/* Record error and continue */
-				netdev_rx_err ( netdev, NULL, -ENOMEM );
-				continue;
-			}
-			memcpy ( iob_put ( pkt, pkt_len ),
-				 ( iobuf->data + pkt_offset ), pkt_len );
-		} else {
-			/* This is the last packet: use in situ */
-			pkt = iob_disown ( iobuf );
-			iob_pull ( pkt, pkt_offset );
-			iob_unput ( pkt, ( iob_len ( pkt ) - pkt_len ) );
+		/* Copy data to a new I/O buffer.  Our USB buffers may
+		 * be very large and so we choose to recycle the
+		 * buffers directly rather than attempt reallocation
+		 * while the device is running.  We therefore copy the
+		 * data to a new I/O buffer even if this is the only
+		 * (or last) packet within the buffer.
+		 */
+		pkt = alloc_iob ( pkt_len );
+		if ( ! pkt ) {
+			/* Record error and continue */
+			netdev_rx_err ( netdev, NULL, -ENOMEM );
+			continue;
 		}
+		memcpy ( iob_put ( pkt, pkt_len ),
+			 ( iobuf->data + pkt_offset ), pkt_len );
 
 		/* Strip CRC, if present */
 		if ( ndp->magic & cpu_to_le32 ( NCM_DATAGRAM_POINTER_MAGIC_CRC))
 		profile_stop ( &ncm_in_datagram_profiler );
 	}
 
-	/* Free I/O buffer (if still present) */
-	free_iob ( iobuf );
-
+	/* Recycle I/O buffer */
+	ncm_rx_recycle ( ncm, &ncm->in, iobuf );
 	profile_stop ( &ncm_in_profiler );
+
 	return;
 
  error:
 	rc = -EIO;
  drop:
+	/* Record error against network device */
 	DBGC_HDA ( ncm, 0, iobuf->data, iob_len ( iobuf ) );
-	netdev_rx_err ( netdev, iobuf, rc );
-	return;
-
+	netdev_rx_err ( netdev, NULL, rc );
  ignore:
-	free_iob ( iobuf );
-	return;
+	ncm_rx_recycle ( ncm, &ncm->in, iobuf );
 }
 
 /** Bulk IN endpoint operations */
 			      struct io_buffer *iobuf ) {
 	struct ncm_ntb_header *header;
 	size_t len = iob_len ( iobuf );
-	size_t header_len = ( sizeof ( *header ) + ncm->padding );
+	size_t header_len = ( sizeof ( *header ) + ncm->out.padding );
 	int rc;
 
 	/* Profile transmissions */
 	/* Populate header */
 	header->nth.magic = cpu_to_le32 ( NCM_TRANSFER_HEADER_MAGIC );
 	header->nth.header_len = cpu_to_le16 ( sizeof ( header->nth ) );
-	header->nth.sequence = cpu_to_le16 ( ncm->sequence );
+	header->nth.sequence = cpu_to_le16 ( ncm->out.sequence );
 	header->nth.len = cpu_to_le16 ( iob_len ( iobuf ) );
 	header->nth.offset =
 		cpu_to_le16 ( offsetof ( typeof ( *header ), ndp ) );
 	memset ( &header->desc[1], 0, sizeof ( header->desc[1] ) );
 
 	/* Enqueue I/O buffer */
-	if ( ( rc = usb_stream ( &ncm->out, iobuf ) ) != 0 )
+	if ( ( rc = usb_stream ( &ncm->out.ep, iobuf ) ) != 0 )
 		return rc;
 
 	/* Increment sequence number */
-	ncm->sequence++;
+	ncm->out.sequence++;
 
 	profile_stop ( &ncm_out_profiler );
 	return 0;
  */
 static void ncm_out_complete ( struct usb_endpoint *ep, struct io_buffer *iobuf,
 			       int rc ) {
-	struct ncm_device *ncm = container_of ( ep, struct ncm_device, out );
+	struct ncm_device *ncm = container_of ( ep, struct ncm_device, out.ep );
 	struct net_device *netdev = ncm->netdev;
 
 	/* Report TX completion */
 	struct ncm_set_ntb_input_size size;
 	int rc;
 
+	/* Allocate I/O buffers */
+	if ( ( rc = ncm_in_alloc ( ncm ) ) != 0 )
+		goto err_alloc;
+
 	/* Set maximum input size */
 	memset ( &size, 0, sizeof ( size ) );
-	size.mtu = cpu_to_le32 ( NCM_NTB_INPUT_SIZE );
+	size.mtu = cpu_to_le32 ( ncm->in.mtu );
 	if ( ( rc = usb_control ( usb, NCM_SET_NTB_INPUT_SIZE, 0, ncm->comms,
 				  &size, sizeof ( size ) ) ) != 0 ) {
-		DBGC ( ncm, "NCM %p could not set input size: %s\n",
-		       ncm, strerror ( rc ) );
+		DBGC ( ncm, "NCM %p could not set input size to %zd: %s\n",
+		       ncm, ncm->in.mtu, strerror ( rc ) );
 		goto err_set_ntb_input_size;
 	}
 
 	}
 
 	/* Open bulk IN endpoint */
-	if ( ( rc = usb_endpoint_open ( &ncm->in ) ) != 0 ) {
+	if ( ( rc = usb_endpoint_open ( &ncm->in.ep ) ) != 0 ) {
 		DBGC ( ncm, "NCM %p could not open bulk IN: %s\n",
 		       ncm, strerror ( rc ) );
 		goto err_open_in;
 	}
 
 	/* Open bulk OUT endpoint */
-	if ( ( rc = usb_endpoint_open ( &ncm->out ) ) != 0 ) {
+	if ( ( rc = usb_endpoint_open ( &ncm->out.ep ) ) != 0 ) {
 		DBGC ( ncm, "NCM %p could not open bulk OUT: %s\n",
 		       ncm, strerror ( rc ) );
 		goto err_open_out;
 	}
 
+	/* Reset transmit sequence number */
+	ncm->out.sequence = 0;
+
 	return 0;
 
-	usb_endpoint_close ( &ncm->out );
+	usb_endpoint_close ( &ncm->out.ep );
  err_open_out:
-	usb_endpoint_close ( &ncm->in );
+	usb_endpoint_close ( &ncm->in.ep );
  err_open_in:
 	usb_set_interface ( usb, ncm->data, 0 );
  err_set_interface:
  err_set_ntb_input_size:
+	ncm_rx_free ( ncm, &ncm->in );
+ err_alloc:
 	return rc;
 }
 
 	struct usb_device *usb = ncm->usb;
 
 	/* Close endpoints */
-	usb_endpoint_close ( &ncm->out );
-	usb_endpoint_close ( &ncm->in );
+	usb_endpoint_close ( &ncm->out.ep );
+	usb_endpoint_close ( &ncm->in.ep );
 
 	/* Reset data interface */
 	usb_set_interface ( usb, ncm->data, 0 );
+
+	/* Free I/O buffers */
+	ncm_rx_free ( ncm, &ncm->in );
 }
 
 /******************************************************************************
 	int rc;
 
 	/* Reset sequence number */
-	ncm->sequence = 0;
+	ncm->out.sequence = 0;
 
 	/* Open communications interface */
 	if ( ( rc = ncm_comms_open ( ncm ) ) != 0 )
 		goto err_comms_open;
 
 	/* Refill interrupt ring */
-	ncm_intr_refill ( ncm );
+	if ( ( rc = ncm_rx_refill ( ncm, &ncm->intr ) ) != 0 )
+		goto err_intr_refill;
 
 	/* Open data interface */
 	if ( ( rc = ncm_data_open ( ncm ) ) != 0 )
 		goto err_data_open;
 
 	/* Refill bulk IN ring */
-	ncm_in_refill ( ncm );
+	if ( ( rc = ncm_rx_refill ( ncm, &ncm->in ) ) != 0 )
+		goto err_in_refill;
 
 	return 0;
 
+ err_in_refill:
 	ncm_data_close ( ncm );
  err_data_open:
+ err_intr_refill:
 	ncm_comms_close ( ncm );
  err_comms_open:
 	return rc;
 
 	/* Close communications interface */
 	ncm_comms_close ( ncm );
-
-	/* Sanity check */
-	assert ( ncm->fill == 0 );
 }
 
 /**
  */
 static void ncm_poll ( struct net_device *netdev ) {
 	struct ncm_device *ncm = netdev->priv;
+	int rc;
 
 	/* Poll USB bus */
 	usb_poll ( ncm->bus );
 
 	/* Refill interrupt ring */
-	ncm_intr_refill ( ncm );
+	if ( ( rc = ncm_rx_refill ( ncm, &ncm->intr ) ) != 0 )
+		netdev_rx_err ( netdev, NULL, rc );
 
 	/* Refill bulk IN ring */
-	ncm_in_refill ( ncm );
+	if ( ( rc = ncm_rx_refill ( ncm, &ncm->in ) ) != 0 )
+		netdev_rx_err ( netdev, NULL, rc );
 }
 
 /** CDC-NCM network device operations */
 	ncm->usb = usb;
 	ncm->bus = usb->port->hub->bus;
 	ncm->netdev = netdev;
-	usb_endpoint_init ( &ncm->intr, usb, &ncm_intr_operations );
-	usb_endpoint_init ( &ncm->in, usb, &ncm_in_operations );
-	usb_endpoint_init ( &ncm->out, usb, &ncm_out_operations );
-	INIT_LIST_HEAD ( &ncm->intrs );
+	usb_endpoint_init ( &ncm->intr.ep, usb, &ncm_intr_operations );
+	usb_endpoint_init ( &ncm->in.ep, usb, &ncm_in_operations );
+	usb_endpoint_init ( &ncm->out.ep, usb, &ncm_out_operations );
 	DBGC ( ncm, "NCM %p on %s\n", ncm, func->name );
 
 	/* Identify interfaces */
 	}
 
 	/* Describe interrupt endpoint */
-	if ( ( rc = usb_endpoint_described ( &ncm->intr, config, comms,
+	if ( ( rc = usb_endpoint_described ( &ncm->intr.ep, config, comms,
 					     USB_INTERRUPT, 0 ) ) != 0 ) {
 		DBGC ( ncm, "NCM %p could not describe interrupt endpoint: "
 		       "%s\n", ncm, strerror ( rc ) );
 	}
 
 	/* Describe bulk IN endpoint */
-	if ( ( rc = usb_endpoint_described ( &ncm->in, config, data,
+	if ( ( rc = usb_endpoint_described ( &ncm->in.ep, config, data,
 					     USB_BULK_IN, 0 ) ) != 0 ) {
 		DBGC ( ncm, "NCM %p could not describe bulk IN endpoint: "
 		       "%s\n", ncm, strerror ( rc ) );
 	}
 
 	/* Describe bulk OUT endpoint */
-	if ( ( rc = usb_endpoint_described ( &ncm->out, config, data,
+	if ( ( rc = usb_endpoint_described ( &ncm->out.ep, config, data,
 					     USB_BULK_OUT, 0 ) ) != 0 ) {
 		DBGC ( ncm, "NCM %p could not describe bulk OUT endpoint: "
 		       "%s\n", ncm, strerror ( rc ) );
 		goto err_ntb_parameters;
 	}
 
+	/* Get maximum supported input size */
+	ncm->mtu = le32_to_cpu ( params.in.mtu );
+	DBGC2 ( ncm, "NCM %p maximum IN size is %zd bytes\n", ncm, ncm->mtu );
+
 	/* Calculate transmit padding */
-	ncm->padding = ( ( le16_to_cpu ( params.out.remainder ) -
-			   sizeof ( struct ncm_ntb_header ) - ETH_HLEN ) &
-			 ( le16_to_cpu ( params.out.divisor ) - 1 ) );
+	ncm->out.padding = ( ( le16_to_cpu ( params.out.remainder ) -
+			       sizeof ( struct ncm_ntb_header ) - ETH_HLEN ) &
+			     ( le16_to_cpu ( params.out.divisor ) - 1 ) );
 	DBGC2 ( ncm, "NCM %p using %zd-byte transmit padding\n",
-		ncm, ncm->padding );
-	assert ( ( ( sizeof ( struct ncm_ntb_header ) + ncm->padding +
+		ncm, ncm->out.padding );
+	assert ( ( ( sizeof ( struct ncm_ntb_header ) + ncm->out.padding +
 		     ETH_HLEN ) % le16_to_cpu ( params.out.divisor ) ) ==
 		 le16_to_cpu ( params.out.remainder ) );
 

src/drivers/net/ncm.h

 	uint32_t mtu;
 } __attribute__ (( packed ));
 
-/** NTB input size
- *
- * This is a policy decision.  2048 is the minimum size which must be
- * supported according to the specification.
- */
-#define NCM_NTB_INPUT_SIZE 2048
+/** Minimum allowed NTB input size */
+#define NCM_MIN_NTB_INPUT_SIZE 2048
+
+/** Maximum allowed NTB input size (16-bit) */
+#define NCM_MAX_NTB_INPUT_SIZE 65536
 
 /** CDC-NCM transfer header (16-bit) */
 struct ncm_transfer_header {
 	struct ncm_datagram_descriptor desc[2];
 } __attribute__ (( packed ));
 
+/** A CDC-NCM receive ring */
+struct ncm_rx_ring {
+	/** USB endpoint */
+	struct usb_endpoint ep;
+	/** I/O buffer size */
+	size_t mtu;
+	/** Recycled buffer list */
+	struct list_head list;
+};
+
+/** A CDC-NCM transmit ring */
+struct ncm_tx_ring {
+	/** USB endpoint */
+	struct usb_endpoint ep;
+	/** Transmitted packet sequence number */
+	uint16_t sequence;
+	/** Alignment padding required on transmitted packets */
+	size_t padding;
+};
+
 /** A CDC-NCM network device */
 struct ncm_device {
 	/** USB device */
 	/** Data interface */
 	unsigned int data;
 
-	/** Interrupt endpoint */
-	struct usb_endpoint intr;
-	/** Bulk IN endpoint */
-	struct usb_endpoint in;
-	/** Bulk OUT endpoint */
-	struct usb_endpoint out;
-
-	/** Recycled interrupt I/O buffers */
-	struct list_head intrs;
-	/** Current bulk IN ring fill level */
-	unsigned int fill;
-	/** Transmitted packet sequence number */
-	uint16_t sequence;
-	/** Alignment padding required on transmitted packets */
-	size_t padding;
+	/** Maximum supported NTB input size */
+	size_t mtu;
+
+	/** Interrupt ring */
+	struct ncm_rx_ring intr;
+	/** Bulk IN ring */
+	struct ncm_rx_ring in;
+	/** Bulk OUT ring */
+	struct ncm_tx_ring out;
 };
 
-/** Bulk IN ring fill level
+/** Bulk IN ring minimum buffer count
+ *
+ * This is a policy decision.
+ */
+#define NCM_IN_MIN_COUNT 3
+
+/** Bulk IN ring minimum total buffer size
+ *
+ * This is a policy decision.
+ */
+#define NCM_IN_MIN_SIZE 16384
+
+/** Bulk IN ring maximum total buffer size
  *
  * This is a policy decision.
  */
-#define NCM_IN_FILL 16
+#define NCM_IN_MAX_SIZE 131072
 
-/** Interrupt ring fill level
+/** Interrupt ring buffer count
  *
  * This is a policy decision.
  */
-#define NCM_INTR_FILL 2
+#define NCM_INTR_COUNT 2
 
 #endif /* _NCM_H */