virtqueue: add writable buffer helper

An application at device side might need to know if a buffer has been
marked as device-writable by the driver.
An example is the virtio SPI protocol, in which the direction of an
half-duplex communication can be determined by the device only by
checking if the buffer containing the data is writable or not.

Add a simple helper to test if a buffer is device-writable, that is,
if the VRING_DESC_F_WRITE flag is present on it.

Signed-off-by: Francesco Valla <francesco@valla.it>

lib/include/openamp/virtqueue.h

@@ -401,6 +401,16 @@ uint32_t virtqueue_get_desc_size(struct virtqueue *vq);
 uint32_t virtqueue_get_buffer_length(struct virtqueue *vq, uint16_t idx);
 void *virtqueue_get_buffer_addr(struct virtqueue *vq, uint16_t idx);
 
+/**
+ * @brief Test if buffer is marked as device-writable
+ *
+ * @param vq	Pointer to VirtIO queue control block
+ * @param idx	Index used in vring desc table
+ *
+ * @return true if the buffer is marked as device-writable, false otherwise
+ */
+bool virtqueue_is_buffer_device_writable(struct virtqueue *vq, uint16_t idx);
+
 /**
  * @brief Test if virtqueue is empty
  *

lib/virtio/virtqueue.c

@@ -187,6 +187,14 @@ void *virtqueue_get_buffer_addr(struct virtqueue *vq, uint16_t idx)
 	return virtqueue_phys_to_virt(vq, vq->vq_ring.desc[idx].addr);
 }
 
+bool virtqueue_is_buffer_device_writable(struct virtqueue *vq, uint16_t idx)
+{
+	/* Invalidate the desc entry written by driver before accessing it */
+	VRING_INVALIDATE(&vq->vq_ring.desc[idx].flags,
+			 sizeof(vq->vq_ring.desc[idx].flags));
+	return !!(vq->vq_ring.desc[idx].flags & VRING_DESC_F_WRITE);
+}
+
 void virtqueue_free(struct virtqueue *vq)
 {
 	if (vq) {