Added first sketch of a generic retry timer mechanism. The idea is to use

these timer objects in AoE and UDP protocols (where there is no underlying
retransmission mechanism) without requiring each protocol to implement its
own individual retry logic.  Eventually, we should be able to use the same
timer code for TCP retransmissions as well.

src/include/gpxe/retry.h

@@ -0,0 +1,36 @@
+#ifndef _GPXE_RETRY_H
+#define _GPXE_RETRY_H
+
+/** @file
+ *
+ * Retry timers
+ *
+ */
+
+#include <gpxe/list.h>
+
+/** Effective maximum retry count for exponential backoff calculation */
+#define BACKOFF_LIMIT 5
+
+/** A retry timer */
+struct retry_timer {
+	/** List of active timers */
+	struct list_head list;
+	/** Base timeout (in ticks) */
+	unsigned int base;
+	/** Retry count */
+	unsigned int retries;
+	/** Expiry time (in ticks) */
+	unsigned long expiry;
+	/** Timer expired callback
+	 *
+	 * @v timer	Retry timer
+	 */
+	void ( * expired ) ( struct retry_timer *timer );
+};
+
+extern void start_timer ( struct retry_timer *timer );
+extern void reset_timer ( struct retry_timer *timer );
+extern void stop_timer ( struct retry_timer *timer );
+
+#endif /* _GPXE_RETRY_H */