stream: Add flow control API

Add basic flow control to stream. A stream slave may return short, indicating
that it is not capable of accepting any more data at the present time. Polling
or a callback can be used via the can_push() function to determine when the
slave can receive again.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>

1 files changed, 30 insertions, 3 deletions

diff --git a/include/hw/stream.h b/include/hw/stream.h
index f6137d6e25..5bc47a93fe 100644
--- a/include/hw/stream.h
+++ b/include/hw/stream.h
@@ -18,14 +18,41 @@ typedef struct StreamSlave {
     Object Parent;
 } StreamSlave;
 
+typedef void (*StreamCanPushNotifyFn)(void *opaque);
+
 typedef struct StreamSlaveClass {
     InterfaceClass parent;
-
-    void (*push)(StreamSlave *obj, unsigned char *buf, size_t len,
+    /**
+     * can push - determine if a stream slave is capable of accepting at least
+     * one byte of data. Returns false if cannot accept. If not implemented, the
+     * slave is assumed to always be capable of recieveing.
+     * @notify: Optional callback that the slave will call when the slave is
+     * capable of recieving again. Only called if false is returned.
+     * @notify_opaque: opaque data to pass to notify call.
+     */
+    bool (*can_push)(StreamSlave *obj, StreamCanPushNotifyFn notify,
+                     void *notify_opaque);
+    /**
+     * push - push data to a Stream slave. The number of bytes pushed is
+     * returned. If the slave short returns, the master must wait before trying
+     * again, the slave may continue to just return 0 waiting for the vm time to
+     * advance. The can_push() function can be used to trap the point in time
+     * where the slave is ready to recieve again, otherwise polling on a QEMU
+     * timer will work.
+     * @obj: Stream slave to push to
+     * @buf: Data to write
+     * @len: Maximum number of bytes to write
+     */
+    size_t (*push)(StreamSlave *obj, unsigned char *buf, size_t len,
                                                     uint32_t *app);
 } StreamSlaveClass;
 
-void
+size_t
 stream_push(StreamSlave *sink, uint8_t *buf, size_t len, uint32_t *app);
 
+bool
+stream_can_push(StreamSlave *sink, StreamCanPushNotifyFn notify,
+                void *notify_opaque);
+
+
 #endif /* STREAM_H */