Added generic asynchronous operations code.

Removed data_in_len and data_out_len from ata_command structure; the
lengths are implied by the sector count and the presence of the data_in
or data_out pointers.

Changed AoE code to use subcommands by default, and made aoe_issue()
nonblocking (with completion via async_wait()).

src/include/gpxe/aoe.h

@@ -10,6 +10,8 @@
 #include <stdint.h>
 #include <gpxe/list.h>
 #include <gpxe/if_ether.h>
+#include <gpxe/retry.h>
+#include <gpxe/async.h>
 #include <gpxe/ata.h>
 
 /** An AoE ATA command */
@@ -89,29 +91,31 @@ struct aoe_session {
 	/** Target MAC address */
 	uint8_t target[ETH_ALEN];
 
-	/** Tag for current command */
+	/** Tag for current AoE command */
 	uint32_t tag;
+
 	/** Current ATA command */
 	struct ata_command *command;
-	/** Status of the command */
-	int status;
+	/** Overall status of current ATA command */
+	unsigned int status;
 	/** Byte offset within command's data buffer */
 	unsigned int command_offset;
+	/** Asynchronous operation for this command */
+	struct async_operation aop;
+
+	/** Retransmission timer */
+	struct retry_timer timer;
 };
 
 #define AOE_STATUS_ERR_MASK	0x0f /**< Error portion of status code */ 
 #define AOE_STATUS_PENDING	0x80 /**< Command pending */
-#define AOE_STATUS_UNDERRUN	0x40 /**< Buffer overrun */
-#define AOE_STATUS_OVERRUN	0x20 /**< Buffer underrun */
 
 /** Maximum number of sectors per packet */
 #define AOE_MAX_COUNT 2
 
 extern void aoe_open ( struct aoe_session *aoe );
 extern void aoe_close ( struct aoe_session *aoe );
-extern int aoe_issue ( struct aoe_session *aoe, struct ata_command *command );
-extern int aoe_issue_split ( struct aoe_session *aoe,
-			     struct ata_command *command );
+extern void aoe_issue ( struct aoe_session *aoe, struct ata_command *command );
 
 /** An AoE device */
 struct aoe_device {

src/include/gpxe/async.h

@@ -0,0 +1,62 @@
+#ifndef _GPXE_ASYNC_H
+#define _GPXE_ASYNC_H
+
+/** @file
+ *
+ * Asynchronous operations
+ *
+ */
+
+#include <errno.h>
+#include <assert.h>
+
+/** An asynchronous operation */
+struct async_operation {
+	/** Operation status
+	 *
+	 * This is an error code as defined in errno.h, plus an offset
+	 * of EINPROGRESS.  This means that a status value of 0
+	 * corresponds to a return status code of -EINPROGRESS,
+	 * i.e. that the default state of an asynchronous operation is
+	 * "not yet completed".
+	 */
+	int status;
+};
+
+/**
+ * Set asynchronous operation status
+ *
+ * @v aop	Asynchronous operation
+ * @v rc	Return status code
+ */
+static inline __attribute__ (( always_inline )) void
+async_set_status ( struct async_operation *aop, int rc ) {
+	aop->status = ( rc + EINPROGRESS );
+}
+
+/**
+ * Get asynchronous operation status
+ *
+ * @v aop	Asynchronous operation
+ * @ret rc	Return status code
+ */
+static inline __attribute__ (( always_inline )) int
+async_status ( struct async_operation *aop ) {
+	return ( aop->status - EINPROGRESS );
+}
+
+/**
+ * Flag asynchronous operation as complete
+ *
+ * @v aop	Asynchronous operation
+ * @v rc	Return status code
+ */
+static inline __attribute__ (( always_inline )) void
+async_done ( struct async_operation *aop, int rc ) {
+	assert ( rc != -EINPROGRESS );
+	async_set_status ( aop, rc );
+}
+
+extern int async_wait ( struct async_operation *aop );
+
+#endif /* _GPXE_ASYNC_H */

src/include/gpxe/ata.h

@@ -139,20 +139,18 @@ struct ata_cb {
 struct ata_command {
 	/** ATA command block */
 	struct ata_cb cb;
-	/** Data-out buffer (may be NULL) */
+	/** Data-out buffer (may be NULL)
+	 *
+	 * If non-NULL, this buffer must be ata_command::cb::count
+	 * sectors in size.
+	 */
 	userptr_t data_out;
-	/** Data-out buffer length
+	/** Data-in buffer (may be NULL)
 	 *
-	 * Must be zero if @c data_out is NULL
+	 * If non-NULL, this buffer must be ata_command::cb::count
+	 * sectors in size.
 	 */
-	size_t data_out_len;
-	/** Data-in buffer (may be NULL) */
 	userptr_t data_in;
-	/** Data-in buffer length
-	 *
-	 * Must be zero if @c data_in is NULL
-	 */
-	size_t data_in_len;
 };
 
 /**