usb: gadget: ncm: Add support to update wMaxSegmentSize via configfs

The max segment size is currently limited to the ethernet frame length of
the kernel which happens to be 1514 at this point in time. However the NCM
specification limits it to 64K for sixtenn bit NTB's. For peer to peer
connections, increasing the segment size gives better throughput.

Add support to configure this value before configfs symlink is created.
Also since the NTB Out/In buffer sizes are fixed at 16384 bytes, limit the
segment size to an upper cap of 8000 to allow at least a minimum of 2 MTU
sized datagrams to be aggregated.

Set the default MTU size for the ncm interface during function bind before
network interface is registered allowing MTU to be set in parity with
wMaxSegmentSize.

Update gadget documentation describing the new configfs property.

Signed-off-by: Krishna Kurapati <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/usb/gadget-testing.rst

···
 The function name to use when creating the function directory is "ncm".
 The NCM function provides these attributes in its function directory:
 
-	=============== ==================================================
-	ifname		network device interface name associated with this
-			function instance
-	qmult		queue length multiplier for high and super speed
-	host_addr	MAC address of host's end of this
-			Ethernet over USB link
-	dev_addr	MAC address of device's end of this
-			Ethernet over USB link
-	=============== ==================================================
+	===============   ==================================================
+	ifname		  network device interface name associated with this
+			  function instance
+	qmult		  queue length multiplier for high and super speed
+	host_addr	  MAC address of host's end of this
+			  Ethernet over USB link
+	dev_addr	  MAC address of device's end of this
+			  Ethernet over USB link
+	max_segment_size  Segment size required for P2P connections. This
+			  will set MTU to (max_segment_size - 14 bytes)
+	===============   ==================================================
 
 and after creating the functions/ncm.<instance name> they contain default
 values: qmult is 5, dev_addr and host_addr are randomly selected.

drivers/usb/gadget/function/f_ncm.c

···
 /* Delay for the transmit to wait before sending an unfilled NTB frame. */
 #define TX_TIMEOUT_NSECS	300000
 
+/*
+ * Although max mtu as dictated by u_ether is 15412 bytes, setting
+ * max_segment_sizeto 15426 would not be efficient. If user chooses segment
+ * size to be (>= 8192), then we can't aggregate more than one  buffer in each
+ * NTB (assuming each packet coming from network layer is >= 8192 bytes) as ep
+ * maxpacket limit is 16384. So let max_segment_size be limited to 8000 to allow
+ * at least 2 packets to be aggregated reducing wastage of NTB buffer space
+ */
+#define MAX_DATAGRAM_SIZE	8000
+
 #define FORMATS_SUPPORTED	(USB_CDC_NCM_NTB16_SUPPORTED |	\
 				 USB_CDC_NCM_NTB32_SUPPORTED)
 
···
 	/* this descriptor actually adds value, surprise! */
 	/* .iMACAddress = DYNAMIC */
 	.bmEthernetStatistics =	cpu_to_le32(0), /* no statistics */
-	.wMaxSegmentSize =	cpu_to_le16(ETH_FRAME_LEN),
 	.wNumberMCFilters =	cpu_to_le16(0),
 	.bNumberPowerFilters =	0,
 };
···
 	struct sk_buff	*skb2;
 	int		ret = -EINVAL;
 	unsigned	ntb_max = le32_to_cpu(ntb_parameters.dwNtbOutMaxSize);
-	unsigned	frame_max = le16_to_cpu(ecm_desc.wMaxSegmentSize);
+	unsigned	frame_max;
 	const struct ndp_parser_opts *opts = ncm->parser_opts;
 	unsigned	crc_len = ncm->is_crc ? sizeof(uint32_t) : 0;
 	int		dgram_counter;
 	int		to_process = skb->len;
+	struct f_ncm_opts *ncm_opts;
+
+	ncm_opts = container_of(port->func.fi, struct f_ncm_opts, func_inst);
+	frame_max = ncm_opts->max_segment_size;
 
 parse_ntb:
 	tmp = (__le16 *)ntb_ptr;
···
 
 	mutex_lock(&ncm_opts->lock);
 	gether_set_gadget(ncm_opts->net, cdev->gadget);
-	if (!ncm_opts->bound)
+	if (!ncm_opts->bound) {
+		ncm_opts->net->mtu = (ncm_opts->max_segment_size - ETH_HLEN);
 		status = gether_register_netdev(ncm_opts->net);
+	}
 	mutex_unlock(&ncm_opts->lock);
 
 	if (status)
···
 	ncm_data_nop_intf.bInterfaceNumber = status;
 	ncm_data_intf.bInterfaceNumber = status;
 	ncm_union_desc.bSlaveInterface0 = status;
+
+	ecm_desc.wMaxSegmentSize = ncm_opts->max_segment_size;
 
 	status = -ENODEV;
 
···
 /* f_ncm_opts_ifname */
 USB_ETHERNET_CONFIGFS_ITEM_ATTR_IFNAME(ncm);
 
+static ssize_t ncm_opts_max_segment_size_show(struct config_item *item,
+					      char *page)
+{
+	struct f_ncm_opts *opts = to_f_ncm_opts(item);
+	u16 segment_size;
+
+	mutex_lock(&opts->lock);
+	segment_size = opts->max_segment_size;
+	mutex_unlock(&opts->lock);
+
+	return sysfs_emit(page, "%u\n", segment_size);
+}
+
+static ssize_t ncm_opts_max_segment_size_store(struct config_item *item,
+					       const char *page, size_t len)
+{
+	struct f_ncm_opts *opts = to_f_ncm_opts(item);
+	u16 segment_size;
+	int ret;
+
+	mutex_lock(&opts->lock);
+	if (opts->refcnt) {
+		ret = -EBUSY;
+		goto out;
+	}
+
+	ret = kstrtou16(page, 0, &segment_size);
+	if (ret)
+		goto out;
+
+	if (segment_size > MAX_DATAGRAM_SIZE) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	opts->max_segment_size = segment_size;
+	ret = len;
+out:
+	mutex_unlock(&opts->lock);
+	return ret;
+}
+
+CONFIGFS_ATTR(ncm_opts_, max_segment_size);
+
 static struct configfs_attribute *ncm_attrs[] = {
 	&ncm_opts_attr_dev_addr,
 	&ncm_opts_attr_host_addr,
 	&ncm_opts_attr_qmult,
 	&ncm_opts_attr_ifname,
+	&ncm_opts_attr_max_segment_size,
 	NULL,
 };
 
···
 		kfree(opts);
 		return ERR_CAST(net);
 	}
+	opts->max_segment_size = cpu_to_le16(ETH_FRAME_LEN);
 	INIT_LIST_HEAD(&opts->ncm_os_desc.ext_prop);
 
 	descs[0] = &opts->ncm_os_desc;

drivers/usb/gadget/function/u_ncm.h

···
 	 */
 	struct mutex			lock;
 	int				refcnt;
+
+	u16				max_segment_size;
 };
 
 #endif /* U_NCM_H */