net: wwan: core: implement TIOCINQ ioctl

It is quite common for a userpace program to fetch the buffered amount
of data in the rx queue to avoid the read block. Implement the TIOCINQ
ioctl to make the migration to the WWAN port usage smooth.

Despite the fact that the read call will return no more data than the
size of a first skb in the queue, TIOCINQ returns the entire amount of
buffered data (sum of all queued skbs). This is done to prevent the
breaking of programs that optimize reading, avoiding it if the buffered
amount of data is too small.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wwan/wwan_core.c

@@ -12,6 +12,7 @@
 #include <linux/skbuff.h>
 #include <linux/slab.h>
 #include <linux/types.h>
+#include <linux/termios.h>
 #include <linux/wwan.h>
 
 /* Maximum number of minors in use */
@@ -618,6 +619,30 @@ static __poll_t wwan_port_fops_poll(struct file *filp, poll_table *wait)
 	return mask;
 }
 
+static long wwan_port_fops_ioctl(struct file *filp, unsigned int cmd,
+				 unsigned long arg)
+{
+	struct wwan_port *port = filp->private_data;
+
+	switch (cmd) {
+	case TIOCINQ: {	/* aka SIOCINQ aka FIONREAD */
+		unsigned long flags;
+		struct sk_buff *skb;
+		int amount = 0;
+
+		spin_lock_irqsave(&port->rxq.lock, flags);
+		skb_queue_walk(&port->rxq, skb)
+			amount += skb->len;
+		spin_unlock_irqrestore(&port->rxq.lock, flags);
+
+		return put_user(amount, (int __user *)arg);
+	}
+
+	default:
+		return -ENOIOCTLCMD;
+	}
+}
+
 static const struct file_operations wwan_port_fops = {
 	.owner = THIS_MODULE,
 	.open = wwan_port_fops_open,
@@ -625,6 +650,10 @@ static const struct file_operations wwan_port_fops = {
 	.read = wwan_port_fops_read,
 	.write = wwan_port_fops_write,
 	.poll = wwan_port_fops_poll,
+	.unlocked_ioctl = wwan_port_fops_ioctl,
+#ifdef CONFIG_COMPAT
+	.compat_ioctl = compat_ptr_ioctl,
+#endif
 	.llseek = noop_llseek,
 };