virtqueue: add virtqueue_get_next_avail_buffer() API

In virtio device side, we always need to get the next avaiable
buffer based on current buffer index. So add this API for
convinience use.

For example, virtio blk driver origanize the buffer:
+----------+
| Reqeust  | (Flags: Read | Next)
+----------+
| Buffer   | (Flags: Read/Write | Next)
+----------+
| Response | (Flags: Write)
+----------+

For the virtio blk device size, we need get the Buffer and Response buffer
based on the Request buffer index.

Signed-off-by: Bowen Wang <[email protected]>
Signed-off-by: Yongrong Wang <[email protected]>

lib/include/openamp/virtqueue.h

@@ -292,6 +292,21 @@ void *virtqueue_get_buffer(struct virtqueue *vq, uint32_t *len, uint16_t *idx);
 void *virtqueue_get_available_buffer(struct virtqueue *vq, uint16_t *avail_idx,
 				     uint32_t *len);
 
+/**
+ * @internal
+ *
+ * @brief Returns next available buffer from VirtIO queue
+ *
+ * @param vq		Pointer to VirtIO queue control block
+ * @param idx		Index used in vring desc table
+ * @param next_idx	Pointer to index of next buffer in vring desc table
+ * @param next_len	Length of next buffer in vring desc table
+ *
+ * @return Pointer to next available buffer
+ */
+void *virtqueue_get_next_avail_buffer(struct virtqueue *vq, uint16_t idx,
+				      uint16_t *next_idx, uint32_t *next_len)
+
 /**
  * @internal
  *

lib/virtio/virtqueue.c

@@ -231,6 +231,28 @@ void *virtqueue_get_available_buffer(struct virtqueue *vq, uint16_t *avail_idx,
 	return buffer;
 }
 
+void *virtqueue_get_next_avail_buffer(struct virtqueue *vq, uint16_t idx,
+				      uint16_t *next_idx, uint32_t *next_len)
+{
+	FAR void *buffer;
+	uint16_t next;
+
+	VRING_INVALIDATE(vq->vq_ring.desc[idx], sizeof(struct vring_desc));
+	if (((vq->vq_ring.desc[idx].flags) & VRING_DESC_F_NEXT) == 0)
+		return NULL;
+
+	next = vq->vq_ring.desc[idx].next;
+	if (next_idx != NULL)
+		*next_idx = next;
+
+	VRING_INVALIDATE(vq->vq_ring.desc[next], sizeof(struct vring_desc));
+	buffer = virtqueue_phys_to_virt(vq, vq->vq_ring.desc[next].addr);
+	if (next_len != NULL)
+		*next_len = vq->vq_ring.desc[next].len;
+
+	return buffer;
+}
+
 int virtqueue_add_consumed_buffer(struct virtqueue *vq, uint16_t head_idx,
 				  uint32_t len)
 {