X-Git-Url: http://git.tuebingen.mpg.de/?p=paraslash.git;a=blobdiff_plain;f=buffer_tree.c;h=4bcc88abb9e56be5b5c29d7b9b9115ff84dfc009;hp=aff03444cc8cc3d0181812fc677c03bfa132e3b0;hb=37e0dfe7bf7737863c63e515e9ed06c2f479f5a3;hpb=ecd2ef112f101a03dda582401d58080cd7c7c43d

diff --git a/buffer_tree.c b/buffer_tree.c
index aff03444..4bcc88ab 100644
--- a/buffer_tree.c
+++ b/buffer_tree.c
@@ -1,5 +1,11 @@
+/*
+ * Copyright (C) 2009-2012 Andre Noll <maan@systemlinux.org>
+ *
+ * Licensed under the GPL v2. For licencing details see COPYING.
+ */
+
+/** \file buffer_tree.c Buffer tree and buffer pool implementations. */
 #include <regex.h>
-#include <stdbool.h>
 
 #include "para.h"
 #include "list.h"
@@ -24,6 +30,8 @@ struct btr_buffer {
 	int refcount;
 	/* NULL means no buffer pool but a malloced buffer. */
 	struct btr_pool *pool;
+	/* Only relevant if pool is NULL. */
+	bool dont_free;
 };
 
 struct btr_buffer_reference {
@@ -57,8 +65,7 @@ struct btr_node {
  * Create a new buffer pool.
  *
  * \param name The name of the new buffer pool.
- *
- * \param area The size in bytes of the pool area.
+ * \param area_size The size in bytes of the pool area.
  *
  * \return An opaque pointer to the newly created buffer pool. It must be
  * passed to btr_pool_free() after it is no longer used to deallocate all
@@ -79,7 +86,7 @@ struct btr_pool *btr_pool_new(const char *name, size_t area_size)
 }
 
 /**
- * Dellocate resources used by a buffer pool.
+ * Deallocate resources used by a buffer pool.
  *
  * \param btrp A pointer obtained via btr_pool_new().
  */
@@ -105,7 +112,7 @@ size_t btr_pool_size(struct btr_pool *btrp)
 	return btrp->area_end - btrp->area_start;
 }
 
-size_t btr_pool_filled(struct btr_pool *btrp)
+static size_t btr_pool_filled(struct btr_pool *btrp)
 {
 	if (!btrp->whead)
 		return btr_pool_size(btrp);
@@ -135,7 +142,7 @@ size_t btr_pool_unused(struct btr_pool *btrp)
  * Return maximal size available for one read. This is
  * smaller than the value returned by btr_pool_unused().
  */
-size_t btr_pool_available(struct btr_pool *btrp)
+static size_t btr_pool_available(struct btr_pool *btrp)
 {
 	if (!btrp->whead)
 		return 0;
@@ -160,6 +167,35 @@ size_t btr_pool_get_buffer(struct btr_pool *btrp, char **result)
 	return btr_pool_available(btrp);
 }
 
+/**
+ * Get references to buffers pointing to free space of the buffer pool area.
+ *
+ * \param btrp The buffer pool.
+ * \param iov The scatter array.
+ *
+ * \return Zero if the buffer pool is full, one if the free space of the buffer
+ * pool area is available as a single contiguous buffer, two if the free space
+ * consists of two buffers. If this function returns the value n, then n
+ * elements of \a iov are initialized.
+ */
+int btr_pool_get_buffers(struct btr_pool *btrp, struct iovec iov[2])
+{
+	size_t sz, unused;
+	char *buf;
+
+	sz = btr_pool_get_buffer(btrp, &buf);
+	if (sz == 0)
+		return 0;
+	iov[0].iov_len = sz;
+	iov[0].iov_base = buf;
+	unused = btr_pool_unused(btrp);
+	if (sz == unused)
+		return 1;
+	iov[1].iov_len = unused - sz;
+	iov[1].iov_base = btrp->area_start;
+	return 2;
+}
+
 /**
  * Mark a part of the buffer pool area as allocated.
  *
@@ -222,9 +258,11 @@ static void btr_pool_deallocate(struct btr_pool *btrp, size_t size)
  *
  * \param bnd Specifies how to create the new node.
  *
- * This function always succeeds (or calls exit()). The returned pointer
- * must be freed using btr_free_node() after it has been removed from
- * the buffer tree via btr_remove_node().
+ * \return A pointer to the newly allocated node.
+ *
+ * This function always succeeds (or calls exit()). The returned pointer must
+ * be freed using btr_free_node() after the node has been removed from the
+ * buffer tree via btr_remove_node().
  */
 struct btr_node *btr_new_node(struct btr_node_description *bnd)
 {
@@ -283,7 +321,7 @@ static void dealloc_buffer(struct btr_buffer *btrb)
 {
 	if (btrb->pool)
 		btr_pool_deallocate(btrb->pool, btrb->size);
-	else
+	else if (!btrb->dont_free)
 		free(btrb->buf);
 }
 
@@ -356,6 +394,34 @@ void btr_add_output(char *buf, size_t size, struct btr_node *btrn)
 	add_btrb_to_children(btrb, btrn, 0);
 }
 
+/**
+ * Insert a buffer into the buffer tree, non-freeing variant.
+ *
+ * \param buf See \ref btr_add_output().
+ * \param size See \ref btr_add_output().
+ * \param btrn See \ref btr_add_output().
+ *
+ * This is similar to btr_add_output() but additionally sets the \p dont_free
+ * flag on \a buf. If the refcount for the buffer drops to zero, \a buf will
+ * not be deallocated if this flag is set.
+ *
+ * The \p dont_free bit also prevents the children of \a btrn from modifying
+ * the buffer contents inplace. Specifically, \ref btr_inplace_ok() returns
+ * false if there is any buffer in the input queue with the \p dont_free bit
+ * set.
+ */
+void btr_add_output_dont_free(const char *buf, size_t size, struct btr_node *btrn)
+{
+	struct btr_buffer *btrb;
+
+	assert(size != 0);
+	if (list_empty(&btrn->children))
+		return;
+	btrb = new_btrb((char *)buf, size);
+	btrb->dont_free = true;
+	add_btrb_to_children(btrb, btrn, 0);
+}
+
 /**
  * Feed data to child nodes of a buffer tree node.
  *
@@ -424,6 +490,17 @@ static void btr_pushdown_br(struct btr_buffer_reference *br, struct btr_node *bt
 	btr_drop_buffer_reference(br);
 }
 
+/**
+ * Feed all buffer references of the input queue through the output channel.
+ *
+ * \param btrn The node whose buffer references should be pushed down.
+ *
+ * This function is useful for filters that do not change the contents of the
+ * buffers at all, like the wav filter or the amp filter if no amplification
+ * was specified. This function is rather cheap.
+ *
+ * \sa \ref btr_pushdown_one().
+ */
 void btr_pushdown(struct btr_node *btrn)
 {
 	struct btr_buffer_reference *br, *tmp;
@@ -432,15 +509,22 @@ void btr_pushdown(struct btr_node *btrn)
 		btr_pushdown_br(br, btrn);
 }
 
-int btr_pushdown_one(struct btr_node *btrn)
+/**
+ * Feed the next buffer of the input queue through the output channel.
+ *
+ * \param btrn The node whose first input queue buffer should be pushed down.
+ *
+ * This works like \ref btr_pushdown() but pushes down only one buffer
+ * reference.
+ */
+void btr_pushdown_one(struct btr_node *btrn)
 {
 	struct btr_buffer_reference *br;
 
 	if (list_empty(&btrn->input_queue))
-		return 0;
+		return;
 	br = list_first_entry(&btrn->input_queue, struct btr_buffer_reference, node);
 	btr_pushdown_br(br, btrn);
-	return 1;
 }
 
 /*
@@ -470,11 +554,35 @@ bool btr_no_parent(struct btr_node *btrn)
 	return !btrn->parent;
 }
 
+/**
+ * Find out whether it is OK to change an input buffer.
+ *
+ * \param btrn The buffer tree node to check.
+ *
+ * This is used by filters that produce exactly the same amount of output as
+ * there is input. The amp filter which multiplies each sample by some number
+ * is an example of such a filter. If there are no other nodes in the buffer
+ * tree that read the same input stream (i.e. if \a btrn has no siblings), a
+ * node may modify its input buffer directly and push down the modified buffer
+ * to its children, thereby avoiding to allocate a possibly large additional
+ * buffer.
+ *
+ * Since the buffer tree may change at any time, this function should be called
+ * during each post_select call.
+ *
+ * \return True if \a btrn has no siblings.
+ */
 bool btr_inplace_ok(struct btr_node *btrn)
 {
-	if (!btrn->parent)
-		return true;
-	return list_is_singular(&btrn->parent->children);
+	struct btr_buffer_reference *br;
+	FOR_EACH_BUFFER_REF(br, btrn) {
+		struct btr_buffer *btrb = br->btrb;
+		if (btrb->refcount > 1)
+			return false;
+		if (btrb->dont_free == true)
+			return false;
+	}
+	return true;
 }
 
 static inline size_t br_available_bytes(struct btr_buffer_reference *br)
@@ -482,7 +590,7 @@ static inline size_t br_available_bytes(struct btr_buffer_reference *br)
 	return br->btrb->size - br->consumed;
 }
 
-size_t btr_get_buffer_by_reference(struct btr_buffer_reference *br, char **buf)
+static size_t btr_get_buffer_by_reference(struct btr_buffer_reference *br, char **buf)
 {
 	if (buf)
 		*buf = br->btrb->buf + br->consumed;
@@ -490,40 +598,95 @@ size_t btr_get_buffer_by_reference(struct btr_buffer_reference *br, char **buf)
 }
 
 /**
- * Obtain the next buffer of the input queue of a buffer tree node.
+ * Obtain the next buffer of the input queue, omitting data.
  *
  * \param btrn The node whose input queue is to be queried.
- * \param bufp Result pointer.
+ * \param omit Number of bytes to be omitted.
+ * \param bufp Result pointer. It is OK to pass \p NULL here.
+ *
+ * If a buffer tree node needs more input data but can not consume the data it
+ * already has (because it might be needed again later) this function can be
+ * used instead of btr_next_buffer() to get a reference to the buffer obtained
+ * by skipping the given number of bytes. Skipped input bytes are not consumed.
  *
- * \return The number of bytes that can be read from buf. Zero if the input
- * buffer queue is empty. In this case the value of \a bufp is undefined.
+ * With a zero \a omit argument, this function is equivalent to \ref
+ * btr_next_buffer().
+ *
+ * \return Number of bytes in \a bufp. If there are less than or equal to \a
+ * omit many bytes available in the input queue of the buffer tree node pointed
+ * to by \a btrn, the function returns zero and the value of \a bufp is
+ * undefined.
  */
-size_t btr_next_buffer(struct btr_node *btrn, char **bufp)
+size_t btr_next_buffer_omit(struct btr_node *btrn, size_t omit, char **bufp)
 {
 	struct btr_buffer_reference *br;
+	size_t wrap_count, sz, rv = 0;
 	char *buf, *result = NULL;
-	size_t sz, rv = 0;
 
-	FOR_EACH_BUFFER_REF(br, btrn) {
+	br = get_first_input_br(btrn);
+	if (!br)
+		return 0;
+	wrap_count = br->wrap_count;
+	if (wrap_count > 0) { /* we have a wrap buffer */
 		sz = btr_get_buffer_by_reference(br, &buf);
-		if (!result) {
-			result = buf;
-			rv = sz;
-			if (!br->btrb->pool)
-				break;
-			continue;
+		if (sz > omit) { /* and it's big enough */
+			result = buf + omit;
+			rv = sz - omit;
+			/*
+			 * Wrap buffers are allocated by malloc(), so the next
+			 * buffer ref will not align nicely, so we return the
+			 * tail of the wrap buffer.
+			 */
+			goto out;
 		}
-		if (!br->btrb->pool)
-			break;
-		if (result + rv != buf)
-			break;
-		rv += sz;
+		/*
+		 * The next wrap_count bytes exist twice, in the wrap buffer
+		 * and as a buffer reference in the buffer tree pool.
+		 */
+		omit += wrap_count;
 	}
+	/*
+	 * For buffer tree pools, the buffers in the list align, i.e. the next
+	 * buffer in the list starts directly at the end of its predecessor. In
+	 * this case we merge adjacent buffers and return one larger buffer
+	 * instead.
+	 */
+	FOR_EACH_BUFFER_REF(br, btrn) {
+		sz = btr_get_buffer_by_reference(br, &buf);
+		if (result) {
+			if (result + rv != buf)
+				goto out;
+			rv += sz;
+		} else if (sz > omit) {
+			result = buf + omit;
+			rv = sz - omit;
+		} else
+			omit -= sz;
+	}
+	if (!result)
+		return 0;
+out:
 	if (bufp)
 		*bufp = result;
 	return rv;
 }
 
+/**
+ * Obtain the next buffer of the input queue of a buffer tree node.
+ *
+ * \param btrn The node whose input queue is to be queried.
+ * \param bufp Result pointer.
+ *
+ * \return The number of bytes that can be read from buf.
+ *
+ * The call of this function is is equivalent to calling \ref
+ * btr_next_buffer_omit() with an \a omit value of zero.
+ */
+size_t btr_next_buffer(struct btr_node *btrn, char **bufp)
+{
+	return btr_next_buffer_omit(btrn, 0, bufp);
+}
+
 /**
  * Deallocate the given number of bytes from the input queue.
  *
@@ -532,7 +695,7 @@ size_t btr_next_buffer(struct btr_node *btrn, char **bufp)
  *
  * This function must be used to get rid of existing buffer references in the
  * node's input queue. If no references to a buffer remain, the underlying
- * buffers are either freed (in the non-buffer tree case) or the read head of
+ * buffers are either freed (in the non-buffer pool case) or the read head of
  * the buffer pool is being advanced.
  *
  * Note that \a numbytes may be smaller than the buffer size. In this case the
@@ -563,7 +726,7 @@ void btr_consume(struct btr_node *btrn, size_t numbytes)
 			numbytes -= br->btrb->size - br->consumed;
 			btr_drop_buffer_reference(br);
 		}
-		assert(true);
+		assert(false);
 	}
 	/*
 	 * We have a wrap buffer, consume from it. If in total, i.e. including
@@ -589,33 +752,50 @@ void btr_consume(struct btr_node *btrn, size_t numbytes)
 	return btr_consume(btrn, sz);
 }
 
-static void flush_input_queue(struct btr_node *btrn)
+/**
+ * Clear the input queue of a buffer tree node.
+ *
+ * \param btrn The node whose input queue should be cleared.
+ */
+void btr_drain(struct btr_node *btrn)
 {
 	struct btr_buffer_reference *br, *tmp;
+
 	FOR_EACH_BUFFER_REF_SAFE(br, tmp, btrn)
 		btr_drop_buffer_reference(br);
 }
 
-void btr_free_node(struct btr_node *btrn)
-{
-	if (!btrn)
-		return;
-	free(btrn->name);
-	free(btrn);
-}
-
-void btr_remove_node(struct btr_node *btrn)
+/**
+ * Remove a node from a buffer tree.
+ *
+ * \param btrnp Determines the node to remove.
+ *
+ * This orphans all children of the node given by \a btrnp and removes this
+ * node from the child list of its parent. Moreover, the input queue is flushed
+ * and the node pointer given by \a btrp is set to \p NULL.
+ *
+ * \sa \ref btr_splice_out_node.
+ */
+void btr_remove_node(struct btr_node **btrnp)
 {
 	struct btr_node *ch;
+	struct btr_node *btrn;
 
-	if (!btrn)
+	if (!btrnp)
 		return;
-	PARA_NOTICE_LOG("removing btr node %s from buffer tree\n", btrn->name);
+	btrn = *btrnp;
+	if (!btrn)
+		goto out;
+	PARA_INFO_LOG("removing btr node %s from buffer tree\n", btrn->name);
 	FOR_EACH_CHILD(ch, btrn)
 		ch->parent = NULL;
-	flush_input_queue(btrn);
+	btr_drain(btrn);
 	if (btrn->parent)
 		list_del(&btrn->node);
+	free(btrn->name);
+	free(btrn);
+out:
+	*btrnp = NULL;
 }
 
 /**
@@ -646,6 +826,17 @@ size_t btr_get_input_queue_size(struct btr_node *btrn)
 	return size;
 }
 
+/**
+ * Remove a node from the buffer tree, reconnecting parent and children.
+ *
+ * \param btrn The node to splice out.
+ *
+ * This function is used by buffer tree nodes that do not exist during the
+ * whole lifetime of the buffer tree. Unlike btr_remove_node(), calling
+ * btr_splice_out_node() does not split the tree into disconnected components
+ * but reconnects the buffer tree by making all child nodes of \a btrn children
+ * of the parent of \a btrn.
+ */
 void btr_splice_out_node(struct btr_node *btrn)
 {
 	struct btr_node *ch, *tmp;
@@ -663,14 +854,18 @@ void btr_splice_out_node(struct btr_node *btrn)
 			list_move(&ch->node, &btrn->parent->children);
 	}
 	assert(list_empty(&btrn->children));
+	btrn->parent = NULL;
 }
 
 /**
- * Return the size of the largest input queue.
+ * Return number of queued output bytes of a buffer tree node.
+ *
+ * \param btrn The node whose output queue size should be computed.
  *
- * Iterates over all children of the given node.
+ * \return This function iterates over all children of the given node and
+ * returns the size of the largest input queue.
  */
-static size_t btr_bytes_pending(struct btr_node *btrn)
+size_t btr_get_output_queue_size(struct btr_node *btrn)
 {
 	size_t max_size = 0;
 	struct btr_node *ch;
@@ -682,42 +877,52 @@ static size_t btr_bytes_pending(struct btr_node *btrn)
 	return max_size;
 }
 
-int btr_exec(struct btr_node *btrn, const char *command, char **value_result)
-{
-	if (!btrn)
-		return -ERRNO_TO_PARA_ERROR(EINVAL);
-	if (!btrn->execute)
-		return -ERRNO_TO_PARA_ERROR(ENOTSUP);
-	return btrn->execute(btrn, command, value_result);
-}
-
 /**
- * Execute a inter-node command.
+ * Execute an inter-node command on the given node or on a parent node.
+ *
+ * \param btrn The node to start looking.
+ * \param command The command to execute.
+ * \param value_result Additional arguments and result value.
+ *
+ * This function traverses the buffer tree from \a btrn upwards and looks for
+ * the first node that understands \a command. On this node \a command is
+ * executed, and the result is stored in \a value_result.
+ *
+ * \return \p -ENOTSUP if no parent node of \a btrn understands \a command.
+ * Otherwise the return value of the command handler is returned.
+ *
+ * \sa \ref receiver::execute, filter::execute, writer::execute.
  */
 int btr_exec_up(struct btr_node *btrn, const char *command, char **value_result)
 {
 	int ret;
 
 	for (; btrn; btrn = btrn->parent) {
-		struct btr_node *parent = btrn->parent;
-		if (!parent)
-			return -ERRNO_TO_PARA_ERROR(ENOTSUP);
-		if (!parent->execute)
+		if (!btrn->execute)
 			continue;
-		PARA_INFO_LOG("parent: %s, cmd: %s\n", parent->name, command);
-		ret = parent->execute(parent, command, value_result);
+		PARA_INFO_LOG("executing %s on %s\n", command, btrn->name);
+		ret = btrn->execute(btrn, command, value_result);
 		if (ret == -ERRNO_TO_PARA_ERROR(ENOTSUP))
 			continue;
 		if (ret < 0)
 			return ret;
 		if (value_result && *value_result)
-			PARA_NOTICE_LOG("%s(%s): %s\n", command, parent->name,
+			PARA_INFO_LOG("%s(%s): %s\n", command, btrn->name,
 				*value_result);
 		return 1;
 	}
 	return -ERRNO_TO_PARA_ERROR(ENOTSUP);
 }
 
+/**
+ * Obtain the context of a buffer node tree.
+ *
+ * \param btrn The node whose output queue size should be computed.
+ *
+ * \return A pointer to the \a context address specified at node creation time.
+ *
+ * \sa btr_new_node(), struct \ref btr_node_description.
+ */
 void *btr_context(struct btr_node *btrn)
 {
 	return btrn->context;
@@ -741,7 +946,7 @@ static void merge_input_pool(struct btr_node *btrn, size_t dest_size)
 	struct btr_buffer_reference *br, *wbr = NULL;
 	int num_refs; /* including wrap buffer */
 	char *buf, *buf1 = NULL, *buf2 = NULL;
-	size_t sz, sz1 = 0, sz2 = 0, wsz;
+	size_t sz, sz1 = 0, sz2 = 0, wb_consumed = 0;
 
 	br = get_first_input_br(btrn);
 	if (!br || br_available_bytes(br) >= dest_size)
@@ -758,6 +963,7 @@ static void merge_input_pool(struct btr_node *btrn, size_t dest_size)
 			wbr = br;
 			if (sz >= dest_size)
 				return;
+			wb_consumed = br->consumed;
 			continue;
 		}
 		if (!buf1) {
@@ -777,7 +983,7 @@ static void merge_input_pool(struct btr_node *btrn, size_t dest_size)
 		assert(buf2 + sz2 == buf);
 		sz2 += sz;
 next:
-		if (sz1 + sz2 >= dest_size)
+		if (sz1 + sz2 >= dest_size + wb_consumed)
 			break;
 	}
 	if (!buf2) /* nothing to do */
@@ -809,7 +1015,6 @@ next:
 	 * We already have a wrap buffer, but it is too small. It might be
 	 * partially used.
 	 */
-	wsz = br_available_bytes(wbr);
 	if (wbr->wrap_count == sz1 && wbr->btrb->size >= sz1 + sz2) /* nothing we can do about it */
 		return;
 	sz = sz1 + sz2 - wbr->btrb->size; /* amount of new data */
@@ -869,6 +1074,20 @@ static int merge_input(struct btr_node *btrn)
 	return 2;
 }
 
+/**
+ * Combine input queue buffers.
+ *
+ * \param btrn The buffer tree node whose input should be merged.
+ * \param dest_size Stop merging if a buffer of at least this size exists.
+ *
+ * Used to combine as many buffers as needed into a single buffer whose size is
+ * at least \a dest_size. This function is rather cheap in case the parent node
+ * uses buffer pools and rather expensive otherwise.
+ *
+ * Note that if less than \a dest_size bytes are available in total, this
+ * function does nothing and subsequent calls to btr_next_buffer() will still
+ * return a buffer size less than \a dest_size.
+ */
 void btr_merge(struct btr_node *btrn, size_t dest_size)
 {
 	if (need_buffer_pool_merge(btrn))
@@ -884,7 +1103,7 @@ void btr_merge(struct btr_node *btrn, size_t dest_size)
 	}
 }
 
-bool btr_eof(struct btr_node *btrn)
+static bool btr_eof(struct btr_node *btrn)
 {
 	char *buf;
 	size_t len = btr_next_buffer(btrn, &buf);
@@ -892,7 +1111,7 @@ bool btr_eof(struct btr_node *btrn)
 	return (len == 0 && btr_no_parent(btrn));
 }
 
-void log_tree_recursively(struct btr_node *btrn, int loglevel, int depth)
+static void log_tree_recursively(struct btr_node *btrn, int loglevel, int depth)
 {
 	struct btr_node *ch;
 	const char spaces[] = "                 ", *space = spaces + 16 - depth;
@@ -904,13 +1123,26 @@ void log_tree_recursively(struct btr_node *btrn, int loglevel, int depth)
 		log_tree_recursively(ch, loglevel, depth + 1);
 }
 
+/**
+ * Write the current buffer (sub-)tree to the log.
+ *
+ * \param btrn Start logging at this node.
+ * \param loglevel Set severity with which the tree should be logged.
+ */
 void btr_log_tree(struct btr_node *btrn, int loglevel)
 {
 	return log_tree_recursively(btrn, loglevel, 0);
 }
 
-/*
- * \return \a root if \a name is \p NULL.
+/**
+ * Find the node with the given name in the buffer tree.
+ *
+ * \param name The name of the node to search.
+ * \param root Where to start the search.
+ *
+ * \return A pointer to the node with the given name on success. If \a name is
+ * \p NULL, the function returns \a root. If there is no node with the given
+ * name, \p NULL is returned.
  */
 struct btr_node *btr_search_node(const char *name, struct btr_node *root)
 {
@@ -929,8 +1161,34 @@ struct btr_node *btr_search_node(const char *name, struct btr_node *root)
 }
 
 /** 640K ought to be enough for everybody ;) */
-#define BTRN_MAX_PENDING (640 * 1024)
+#define BTRN_MAX_PENDING (96 * 1024)
 
+/**
+ * Return the current state of a buffer tree node.
+ *
+ * \param btrn The node whose state should be queried.
+ * \param min_iqs The minimal input queue size.
+ * \param type The supposed type of \a btrn.
+ *
+ * Most users of the buffer tree subsystem call this function from both
+ * their pre_select and the post_select methods.
+ *
+ * \return Negative if an error condition was detected, zero if there
+ * is nothing to do and positive otherwise.
+ *
+ * Examples:
+ *
+ * - If a non-root node has no parent and an empty input queue, the function
+ * returns \p -E_BTR_EOF. Similarly, if a non-leaf node has no children, \p
+ * -E_BTR_NO_CHILD is returned.
+ *
+ * - If less than \a min_iqs many bytes are available in the input queue and no
+ * EOF condition was detected, the function returns zero.
+ *
+ * - If there's plenty of data left in the input queue of the children of \a
+ * btrn, the function also returns zero in order to bound the memory usage of
+ * the buffer tree.
+ */
 int btr_node_status(struct btr_node *btrn, size_t min_iqs,
 	enum btr_node_type type)
 {
@@ -940,7 +1198,7 @@ int btr_node_status(struct btr_node *btrn, size_t min_iqs,
 	if (type != BTR_NT_LEAF) {
 		if (btr_no_children(btrn))
 			return -E_BTR_NO_CHILD;
-		if (btr_bytes_pending(btrn) > BTRN_MAX_PENDING)
+		if (btr_get_output_queue_size(btrn) > BTRN_MAX_PENDING)
 			return 0;
 	}
 	if (type != BTR_NT_ROOT) {
@@ -955,7 +1213,20 @@ int btr_node_status(struct btr_node *btrn, size_t min_iqs,
 	return 1;
 }
 
+/**
+ * Get the time of the first I/O for a buffer tree node.
+ *
+ * \param btrn The node whose I/O time should be obtained.
+ * \param tv Result pointer.
+ *
+ * Mainly useful for the time display of para_audiod.
+ */
 void btr_get_node_start(struct btr_node *btrn, struct timeval *tv)
 {
 	*tv = btrn->start;
 }
+
+struct btr_node *btr_parent(struct btr_node *btrn)
+{
+	return btrn->parent;
+}