wwan: core: Support slicing in port TX flow of WWAN subsystem

wwan_port_fops_write inputs the SKB parameter to the TX callback of
the WWAN device driver. However, the WWAN device (e.g., t7xx) may
have an MTU less than the size of SKB, causing the TX buffer to be
sliced and copied once more in the WWAN device driver.

This patch implements the slicing in the WWAN subsystem and gives
the WWAN devices driver the option to slice(by frag_len) or not. By
doing so, the additional memory copy is reduced.

Meanwhile, this patch gives WWAN devices driver the option to reserve
headroom in fragments for the device-specific metadata.

Signed-off-by: haozhe chang <[e-mail]>
Reviewed-by: Loic Poulain <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/wwan/iosm/iosm_ipc_port.c

···
 	ipc_port->ipc_imem = ipc_imem;
 
 	ipc_port->iosm_port = wwan_create_port(ipc_port->dev, port_type,
-					       &ipc_wwan_ctrl_ops, ipc_port);
+					       &ipc_wwan_ctrl_ops, NULL,
+					       ipc_port);
 
 	return ipc_port;
 }

drivers/net/wwan/mhi_wwan_ctrl.c

···
 
 	/* Register as a wwan port, id->driver_data contains wwan port type */
 	port = wwan_create_port(&cntrl->mhi_dev->dev, id->driver_data,
-				&wwan_pops, mhiwwan);
+				&wwan_pops, NULL, mhiwwan);
 	if (IS_ERR(port)) {
 		kfree(mhiwwan);
 		return PTR_ERR(port);

drivers/net/wwan/rpmsg_wwan_ctrl.c

···
 
 	/* Register as a wwan port, id.driver_data contains wwan port type */
 	port = wwan_create_port(parent, rpdev->id.driver_data,
-				&rpmsg_wwan_pops, rpwwan);
+				&rpmsg_wwan_pops, NULL, rpwwan);
 	if (IS_ERR(port))
 		return PTR_ERR(port);
 

drivers/net/wwan/t7xx/t7xx_port_wwan.c

···
 static int t7xx_port_ctrl_tx(struct wwan_port *port, struct sk_buff *skb)
 {
 	struct t7xx_port *port_private = wwan_port_get_drvdata(port);
-	size_t len, offset, chunk_len = 0, txq_mtu = CLDMA_MTU;
 	const struct t7xx_port_conf *port_conf;
+	struct sk_buff *cur = skb, *cloned;
 	struct t7xx_fsm_ctl *ctl;
 	enum md_state md_state;
+	int cnt = 0, ret;
 
-	len = skb->len;
-	if (!len || !port_private->chan_enable)
+	if (!port_private->chan_enable)
 		return -EINVAL;
 
 	port_conf = port_private->port_conf;
···
 		return -ENODEV;
 	}
 
-	for (offset = 0; offset < len; offset += chunk_len) {
-		struct sk_buff *skb_ccci;
-		int ret;
-
-		chunk_len = min(len - offset, txq_mtu - sizeof(struct ccci_header));
-		skb_ccci = t7xx_port_alloc_skb(chunk_len);
-		if (!skb_ccci)
-			return -ENOMEM;
-
-		skb_put_data(skb_ccci, skb->data + offset, chunk_len);
-		ret = t7xx_port_send_skb(port_private, skb_ccci, 0, 0);
+	while (cur) {
+		cloned = skb_clone(cur, GFP_KERNEL);
+		cloned->len = skb_headlen(cur);
+		ret = t7xx_port_send_skb(port_private, cloned, 0, 0);
 		if (ret) {
-			dev_kfree_skb_any(skb_ccci);
+			dev_kfree_skb(cloned);
 			dev_err(port_private->dev, "Write error on %s port, %d\n",
 				port_conf->name, ret);
-			return ret;
+			return cnt ? cnt + ret : ret;
 		}
+		cnt += cur->len;
+		if (cur == skb)
+			cur = skb_shinfo(skb)->frag_list;
+		else
+			cur = cur->next;
 	}
 
 	dev_kfree_skb(skb);
···
 static void t7xx_port_wwan_md_state_notify(struct t7xx_port *port, unsigned int state)
 {
 	const struct t7xx_port_conf *port_conf = port->port_conf;
+	unsigned int header_len = sizeof(struct ccci_header);
+	struct wwan_port_caps caps;
 
 	if (state != MD_STATE_READY)
 		return;
 
 	if (!port->wwan.wwan_port) {
+		caps.frag_len = CLDMA_MTU - header_len;
+		caps.headroom_len = header_len;
 		port->wwan.wwan_port = wwan_create_port(port->dev, port_conf->port_type,
-							&wwan_ops, port);
+							&wwan_ops, &caps, port);
 		if (IS_ERR(port->wwan.wwan_port))
 			dev_err(port->dev, "Unable to create WWWAN port %s", port_conf->name);
 	}

drivers/net/wwan/wwan_core.c

···
  * @rxq: Buffer inbound queue
  * @waitqueue: The waitqueue for port fops (read/write/poll)
  * @data_lock: Port specific data access serialization
+ * @headroom_len: SKB reserved headroom size
+ * @frag_len: Length to fragment packet
  * @at_data: AT port specific data
  */
 struct wwan_port {
···
 	struct sk_buff_head rxq;
 	wait_queue_head_t waitqueue;
 	struct mutex data_lock;	/* Port specific data access serialization */
+	size_t headroom_len;
+	size_t frag_len;
 	union {
 		struct {
 			struct ktermios termios;
···
 struct wwan_port *wwan_create_port(struct device *parent,
 				   enum wwan_port_type type,
 				   const struct wwan_port_ops *ops,
+				   struct wwan_port_caps *caps,
 				   void *drvdata)
 {
 	struct wwan_device *wwandev;
···
 
 	port->type = type;
 	port->ops = ops;
+	port->frag_len = caps ? caps->frag_len : SIZE_MAX;
+	port->headroom_len = caps ? caps->headroom_len : 0;
 	mutex_init(&port->ops_lock);
 	skb_queue_head_init(&port->rxq);
 	init_waitqueue_head(&port->waitqueue);
···
 static ssize_t wwan_port_fops_write(struct file *filp, const char __user *buf,
 				    size_t count, loff_t *offp)
 {
+	struct sk_buff *skb, *head = NULL, *tail = NULL;
 	struct wwan_port *port = filp->private_data;
-	struct sk_buff *skb;
+	size_t frag_len, remain = count;
 	int ret;
 
 	ret = wwan_wait_tx(port, !!(filp->f_flags & O_NONBLOCK));
 	if (ret)
 		return ret;
 
-	skb = alloc_skb(count, GFP_KERNEL);
-	if (!skb)
-		return -ENOMEM;
+	do {
+		frag_len = min(remain, port->frag_len);
+		skb = alloc_skb(frag_len + port->headroom_len, GFP_KERNEL);
+		if (!skb) {
+			ret = -ENOMEM;
+			goto freeskb;
+		}
+		skb_reserve(skb, port->headroom_len);
 
-	if (copy_from_user(skb_put(skb, count), buf, count)) {
-		kfree_skb(skb);
-		return -EFAULT;
-	}
+		if (!head) {
+			head = skb;
+		} else if (!tail) {
+			skb_shinfo(head)->frag_list = skb;
+			tail = skb;
+		} else {
+			tail->next = skb;
+			tail = skb;
+		}
 
-	ret = wwan_port_op_tx(port, skb, !!(filp->f_flags & O_NONBLOCK));
-	if (ret) {
-		kfree_skb(skb);
-		return ret;
-	}
+		if (copy_from_user(skb_put(skb, frag_len), buf + count - remain, frag_len)) {
+			ret = -EFAULT;
+			goto freeskb;
+		}
 
-	return count;
+		if (skb != head) {
+			head->data_len += skb->len;
+			head->len += skb->len;
+			head->truesize += skb->truesize;
+		}
+	} while (remain -= frag_len);
+
+	ret = wwan_port_op_tx(port, head, !!(filp->f_flags & O_NONBLOCK));
+	if (!ret)
+		return count;
+
+freeskb:
+	kfree_skb(head);
+	return ret;
 }
 
 static __poll_t wwan_port_fops_poll(struct file *filp, poll_table *wait)

drivers/net/wwan/wwan_hwsim.c

···
 
 	port->wwan = wwan_create_port(&dev->dev, WWAN_PORT_AT,
 				      &wwan_hwsim_port_ops,
-				      port);
+				      NULL, port);
 	if (IS_ERR(port->wwan)) {
 		err = PTR_ERR(port->wwan);
 		goto err_free_port;

drivers/usb/class/cdc-wdm.c

···
 		return;
 	}
 
-	port = wwan_create_port(&intf->dev, desc->wwanp_type, &wdm_wwan_port_ops, desc);
+	port = wwan_create_port(&intf->dev, desc->wwanp_type, &wdm_wwan_port_ops,
+				NULL, desc);
 	if (IS_ERR(port)) {
 		dev_err(&intf->dev, "%s: Unable to create WWAN port\n",
 			dev_name(intf->usb_dev));

include/linux/wwan.h

···
 			    poll_table *wait);
 };
 
+/** struct wwan_port_caps - The WWAN port capbilities
+ * @frag_len: WWAN port TX fragments length
+ * @headroom_len: WWAN port TX fragments reserved headroom length
+ */
+struct wwan_port_caps {
+	size_t frag_len;
+	unsigned int headroom_len;
+};
+
 /**
  * wwan_create_port - Add a new WWAN port
  * @parent: Device to use as parent and shared by all WWAN ports
  * @type: WWAN port type
  * @ops: WWAN port operations
+ * @caps: WWAN port capabilities
  * @drvdata: Pointer to caller driver data
  *
  * Allocate and register a new WWAN port. The port will be automatically exposed
···
 struct wwan_port *wwan_create_port(struct device *parent,
 				   enum wwan_port_type type,
 				   const struct wwan_port_ops *ops,
+				   struct wwan_port_caps *caps,
 				   void *drvdata);
 
 /**