[PATCH 2/2] usb: gadget: ncm: Add support to update wMaxSegmentSize via configfs

[Krishna Kurapati <quic_kriskura@xxxxxxxxxxx>]

Currently the NCM driver restricts wMaxSegmentSize that indicates
the datagram size coming from network layer to 1514. However the
spec doesn't have any limitation. For P2P connections over NCM,
increasing MTU helps increasing 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 15014. 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.

Signed-off-by: Krishna Kurapati <quic_kriskura@xxxxxxxxxxx>
---
 drivers/usb/gadget/function/f_ncm.c | 51 +++++++++++++++++++++++++++++
 drivers/usb/gadget/function/u_ncm.h |  2 ++
 2 files changed, 53 insertions(+)

diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
index feccf4c8cc4f..eab297b22200 100644
--- a/drivers/usb/gadget/function/f_ncm.c
+++ b/drivers/usb/gadget/function/f_ncm.c
@@ -103,6 +103,8 @@ static inline struct f_ncm *func_to_ncm(struct usb_function *f)
 /* Delay for the transmit to wait before sending an unfilled NTB frame. */
 #define TX_TIMEOUT_NSECS	300000
 
+#define MAX_DATAGRAM_SIZE	15014
+
 #define FORMATS_SUPPORTED	(USB_CDC_NCM_NTB16_SUPPORTED |	\
 				 USB_CDC_NCM_NTB32_SUPPORTED)
 
@@ -1408,6 +1410,7 @@ static int ncm_bind(struct usb_configuration *c, struct usb_function *f)
 	ncm_opts = container_of(f->fi, struct f_ncm_opts, func_inst);
 
 	if (cdev->use_os_string) {
+		ncm_opts->net->mtu = (ncm_opts->max_segment_size - ETH_HLEN);
 		f->os_desc_table = kzalloc(sizeof(*f->os_desc_table),
 					   GFP_KERNEL);
 		if (!f->os_desc_table)
@@ -1469,6 +1472,8 @@ static int ncm_bind(struct usb_configuration *c, struct usb_function *f)
 
 	status = -ENODEV;
 
+	ecm_desc.wMaxSegmentSize = ncm_opts->max_segment_size;
+
 	/* allocate instance-specific endpoints */
 	ep = usb_ep_autoconfig(cdev->gadget, &fs_ncm_in_desc);
 	if (!ep)
@@ -1569,11 +1574,56 @@ USB_ETHERNET_CONFIGFS_ITEM_ATTR_QMULT(ncm);
 /* 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);
+	u32 segment_size;
+
+	mutex_lock(&opts->lock);
+	segment_size = opts->max_segment_size;
+	mutex_unlock(&opts->lock);
+
+	return sprintf(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);
+	int ret;
+	u32 segment_size;
+
+	mutex_lock(&opts->lock);
+	if (opts->refcnt) {
+		ret = -EBUSY;
+		goto out;
+	}
+
+	ret = kstrtou32(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,
 };
 
@@ -1616,6 +1666,7 @@ static struct usb_function_instance *ncm_alloc_inst(void)
 		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;
diff --git a/drivers/usb/gadget/function/u_ncm.h b/drivers/usb/gadget/function/u_ncm.h
index 5408854d8407..d3403cf13f17 100644
--- a/drivers/usb/gadget/function/u_ncm.h
+++ b/drivers/usb/gadget/function/u_ncm.h
@@ -31,6 +31,8 @@ struct f_ncm_opts {
 	 */
 	struct mutex			lock;
 	int				refcnt;
+
+	u32				max_segment_size;
 };
 
 #endif /* U_NCM_H */
-- 
2.42.0