[RFC v2] usb: gadget: ncm: Add support to configure wMaxSegmentSize

[Krishna Kurapati]

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.

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>
---
Link to v1: https://lore.kernel.org/all/20230212175659.4480-1-quic_kriskura@xxxxxxxxxxx/

Fixed the Max datagram size as per comments on v1.
As per the spec, there are no GSO related specifications present mentioned by Maintainer.
Pushing v2 to fix datagram length specification and take the discussion regarding GSO
forward.


 drivers/usb/gadget/function/f_ncm.c | 54 +++++++++++++++++++++++++++++
 drivers/usb/gadget/function/u_ncm.h |  1 +
 2 files changed, 55 insertions(+)

diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
index 424bb3b666db..a990f407d6c3 100644
--- a/drivers/usb/gadget/function/f_ncm.c
+++ b/drivers/usb/gadget/function/f_ncm.c
@@ -97,6 +97,11 @@ static inline unsigned ncm_bitrate(struct usb_gadget *g)
 
 /*-------------------------------------------------------------------------*/
 
+/*
+ * Provide felxibility to change MTU upto 15000 (as the ETH_HLEN is 14 bytes)
+ */
+#define MAX_DATAGRAM_SIZE	15014
+
 /*
  * We cannot group frames so use just the minimal size which ok to put
  * one max-size ethernet frame.
@@ -1440,6 +1445,7 @@ static int ncm_bind(struct usb_configuration *c, struct usb_function *f)
 	 */
 	if (!ncm_opts->bound) {
 		mutex_lock(&ncm_opts->lock);
+		ncm_opts->net->mtu = (ncm_opts->max_segment_size - ETH_HLEN);
 		gether_set_gadget(ncm_opts->net, cdev->gadget);
 		status = gether_register_netdev(ncm_opts->net);
 		mutex_unlock(&ncm_opts->lock);
@@ -1484,6 +1490,8 @@ 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)
@@ -1586,11 +1594,56 @@ 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,
 };
 
@@ -1633,6 +1686,7 @@ 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..fab99d997476 100644
--- a/drivers/usb/gadget/function/u_ncm.h
+++ b/drivers/usb/gadget/function/u_ncm.h
@@ -31,6 +31,7 @@ struct f_ncm_opts {
 	 */
 	struct mutex			lock;
 	int				refcnt;
+	u32				max_segment_size;
 };
 
 #endif /* U_NCM_H */
-- 
2.39.0