On Fri, 14 Nov 2008 14:34:27 -0700 Dan Williams [off-list ref] wrote:

yuo cnat sepll

+ * @tx: transaction to wait on
+ */
+enum dma_status
+dma_wait_for_async_tx(struct dma_async_tx_descriptor *tx)
+{
+	enum dma_status status;
+	struct dma_async_tx_descriptor *iter;
+	struct dma_async_tx_descriptor *parent;
+
+	if (!tx)
+		return DMA_SUCCESS;
+
+	/* poll through the dependency chain, return when tx is complete */
+	do {
+		iter = tx;
+
+		/* find the root of the unsubmitted dependency chain */
+		do {
+			parent = iter->parent;
+			if (!parent)
+				break;
+			else
+				iter = parent;
+		} while (parent);
+
+		/* there is a small window for ->parent == NULL and
+		 * ->cookie == -EBUSY
+		 */
+		while (iter->cookie == -EBUSY)
+			cpu_relax();
+
+		status = dma_sync_wait(iter->chan, iter->cookie);
+	} while (status == DMA_IN_PROGRESS || (iter != tx));
+
+	return status;
+}
+EXPORT_SYMBOL_GPL(dma_wait_for_async_tx);

hm, strange.

The unlocked list walk assumes that this thread of control has
exclusive access to *tx (somehow??), but this thread of control doesn't
end up freeing *tx.  I guess the caller does that.

+/* dma_run_dependencies - helper routine for dma drivers to process
+ *	(start) dependent operations on their target channel
+ * @tx: transaction with dependencies
+ */
+void dma_run_dependencies(struct dma_async_tx_descriptor *tx)
+{
+	struct dma_async_tx_descriptor *dep = tx->next;
+	struct dma_async_tx_descriptor *dep_next;
+	struct dma_chan *chan;
+
+	if (!dep)
+		return;
+
+	chan = dep->chan;
+
+	/* keep submitting up until a channel switch is detected
+	 * in that case we will be called again as a result of
+	 * processing the interrupt from async_tx_channel_switch
+	 */
+	for (; dep; dep = dep_next) {
+		spin_lock_bh(&dep->lock);
+		dep->parent = NULL;
+		dep_next = dep->next;
+		if (dep_next && dep_next->chan == chan)
+			dep->next = NULL; /* ->next will be submitted */
+		else
+			dep_next = NULL; /* submit current dep and terminate */
+		spin_unlock_bh(&dep->lock);
+
+		dep->tx_submit(dep);
+	}
+
+	chan->device->device_issue_pending(chan);
+}
+EXPORT_SYMBOL_GPL(dma_run_dependencies);

Now there's a lock.