Clarify the usage of TX descriptors

Author: Cuda-Chen

virtio-snd.c

@@ -724,7 +724,6 @@ static int virtio_snd_desc_handler(virtio_snd_state_t *vsnd,
                                    uint32_t desc_idx,
                                    uint32_t *plen)
 {
-    /* TODO: clarify the use of the third descriptor */
     /* virtio-snd command uses at most 3 virtqueue descriptors, where
      * the first descriptor contains:
      *   struct virtio_snd_hdr hdr (for request)
@@ -759,9 +758,6 @@ static int virtio_snd_desc_handler(virtio_snd_state_t *vsnd,
         (virtio_snd_hdr_t *) ((uintptr_t) vsnd->ram + vq_desc[1].addr);
     const void *query = (void *) ((uintptr_t) vsnd->ram + vq_desc[0].addr);
 
-    /* TODO: let the program use this variable selectively according to
-     * the type.
-     */
     /* As there are plenty of structures for response payload,
      * use a void pointer for generic type support.
      */
@@ -840,14 +836,14 @@ static int virtio_snd_tx_desc_handler(virtio_snd_state_t *vsnd,
                                       uint32_t desc_idx,
                                       uint32_t *plen)
 {
-    /* TODO: clarify the usage of the last descriptor. */
-    /* virtio-snd TX uses arbitrary number of virtqueue descriptors.
-     * The first descritor descriptor contains:
-     *   struct virtio_snd_hdr hdr (for request)
-     * Next, the payload consists of one or more descriptors
+    /* A PCM I/O message uses at least 3 virtqueue descriptors to
+     * represent a PCM data of a period size.
+     * The first part contains one descriptor as follows:
+     *   struct virtio_snd_pcm_xfer
+     * The second part contains one or more descriptors
      * representing PCM frames.
-     * Finally, the last descriptors contains:
-     *   (response payload structure)
+     * the last part contains one descriptor as follows:
+     *   struct virtio_snd_pcm_status
      */
     virtq_desc_queue_node_t *node;
     struct list_head q;
@@ -891,7 +887,6 @@ static int virtio_snd_tx_desc_handler(virtio_snd_state_t *vsnd,
                 (virtio_snd_pcm_status_t *) (base + addr);
             response->status = VIRTIO_SND_S_OK;
             response->latency_bytes = ret_len;
-            printf("=== TX queue get %" PRIu32 " bytes ===\n", ret_len);
             *plen = sizeof(*response);
             goto early_continue;
         }
@@ -985,7 +980,7 @@ static void virtio_queue_notify_handler(
 }
 
 /* TX thread context */
-/* Control the callback to prepare the buffer */
+/* Receive PCM frames from driver. */
 static void *func(void *args)
 {
     virtio_snd_state_t *vsnd = (virtio_snd_state_t *) args;