virtio: console: Introduce a send_buf function for a common path for sending data to host

Adding support for generic ports that will write to userspace will need
some code changes.

Consolidate the write routine into send_buf() and put_chars() now just
calls into the new function.
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/virtio_console.c

@@ -233,6 +233,38 @@ static bool port_has_data(struct port *port)
 	return ret;
 }
 
+static ssize_t send_buf(struct port *port, void *in_buf, size_t in_count)
+{
+	struct scatterlist sg[1];
+	struct virtqueue *out_vq;
+	ssize_t ret;
+	unsigned int len;
+
+	out_vq = port->out_vq;
+
+	sg_init_one(sg, in_buf, in_count);
+	ret = out_vq->vq_ops->add_buf(out_vq, sg, 1, 0, in_buf);
+
+	/* Tell Host to go! */
+	out_vq->vq_ops->kick(out_vq);
+
+	if (ret < 0) {
+		len = 0;
+		goto fail;
+	}
+
+	/*
+	 * Wait till the host acknowledges it pushed out the data we
+	 * sent. Also ensure we return to userspace the number of
+	 * bytes that were successfully consumed by the host.
+	 */
+	while (!out_vq->vq_ops->get_buf(out_vq, &len))
+		cpu_relax();
+fail:
+	/* We're expected to return the amount of data we wrote */
+	return len;
+}
+
 /*
  * Give out the data that's requested from the buffer that we have
  * queued up.
@@ -280,10 +312,7 @@ static ssize_t fill_readbuf(struct port *port, char *out_buf, size_t out_count)
  */
 static int put_chars(u32 vtermno, const char *buf, int count)
 {
-	struct scatterlist sg[1];
 	struct port *port;
-	struct virtqueue *out_vq;
-	unsigned int len;
 
 	port = find_port_by_vtermno(vtermno);
 	if (!port)
@@ -292,20 +321,7 @@ static int put_chars(u32 vtermno, const char *buf, int count)
 	if (unlikely(early_put_chars))
 		return early_put_chars(vtermno, buf, count);
 
-	out_vq = port->out_vq;
-	/* This is a convenient routine to initialize a single-elem sg list */
-	sg_init_one(sg, buf, count);
-
-	/* This shouldn't fail: if it does, we lose chars. */
-	if (out_vq->vq_ops->add_buf(out_vq, sg, 1, 0, port) >= 0) {
-		/* Tell Host to go! */
-		out_vq->vq_ops->kick(out_vq);
-		while (!out_vq->vq_ops->get_buf(out_vq, &len))
-			cpu_relax();
-	}
-
-	/* We're expected to return the amount of data we wrote: all of it. */
-	return count;
+	return send_buf(port, (void *)buf, count);
 }
 
 /*