Use separate data-in and data-out buffers.

Increase code simplicity at the expense of around 64 bytes.

src/drivers/block/ata.c

 }
 
 /**
- * Read block from / write block to ATA device
+ * Read block from ATA device
  *
- * @v write		Write flag (ATA_FL_WRITE or 0)
  * @v blockdev		Block device
  * @v block		LBA block number
  * @v count		Block count
  * @v buffer		Data buffer
  * @ret rc		Return status code
  */
-static __attribute__ (( regparm ( 1 ) )) int
-ata_rw ( int write, struct block_device *blockdev, uint64_t block,
-	 unsigned long count, userptr_t buffer ) {
+static int ata_read ( struct block_device *blockdev, uint64_t block,
+		      unsigned long count, userptr_t buffer ) {
 	struct ata_device *ata = block_to_ata ( blockdev );
 	struct ata_command command;
-	int lba48 = ( ata->flags & ATA_FL_LBA48 );
 
 	memset ( &command, 0, sizeof ( command ) );
 	command.cb.lba.native = block;
 	command.cb.count.native = count;
-	command.cb.device = ( ata->flags | ATA_DEV_OBSOLETE | ATA_DEV_LBA );
-	command.cb.flags = ( ata->flags | write );
-	command.cb.cmd_stat = ( write ? ATA_CMD_WRITE : ATA_CMD_READ );
-	if ( lba48 ) {
-		command.cb.cmd_stat |= ATA_CMD_EXT;
-	} else {
+	command.cb.device = ( ata->device | ATA_DEV_OBSOLETE | ATA_DEV_LBA );
+	command.cb.lba48 = ata->lba48;
+	if ( ! ata->lba48 )
 		command.cb.device |= command.cb.lba.bytes.low_prev;
-	}
-	command.data = buffer;
-	command.data_len = ( count * blockdev->blksize );
+	command.cb.cmd_stat = ( ata->lba48 ? ATA_CMD_READ_EXT : ATA_CMD_READ );
+	command.data_in = buffer;
+	command.data_in_len = ( count * blockdev->blksize );
 	return ata_command ( ata, &command );
 }
 
-/**
- * Read block from ATA device
- *
- * @v blockdev		Block device
- * @v block		LBA block number
- * @v count		Block count
- * @v buffer		Data buffer
- * @ret rc		Return status code
- */
-static int ata_read ( struct block_device *blockdev, uint64_t block,
-		      unsigned long count, userptr_t buffer ) {
-	/* Pass through to ata_rw().  Since ata_rw is regparm(1), this
-	 * is extremely efficient; just a mov and a jmp.
-	 */
-	return ata_rw ( 0, blockdev, block, count, buffer );
-}
-
 /**
  * Write block to ATA device
  *
  */
 static int ata_write ( struct block_device *blockdev, uint64_t block,
 		       unsigned long count, userptr_t buffer ) {
-	/* Pass through to ata_rw().  Since ata_rw is regparm(1), this
-	 * is extremely efficient; just a mov and a jmp.
-	 */
-	return ata_rw ( ATA_FL_WRITE, blockdev, block, count, buffer );
+	struct ata_device *ata = block_to_ata ( blockdev );
+	struct ata_command command;
+	
+	memset ( &command, 0, sizeof ( command ) );
+	command.cb.lba.native = block;
+	command.cb.count.native = count;
+	command.cb.device = ( ata->device | ATA_DEV_OBSOLETE | ATA_DEV_LBA );
+	command.cb.lba48 = ata->lba48;
+	if ( ! ata->lba48 )
+		command.cb.device |= command.cb.lba.bytes.low_prev;
+	command.cb.cmd_stat = ( ata->lba48 ?
+				ATA_CMD_WRITE_EXT : ATA_CMD_WRITE );
+	command.data_out = buffer;
+	command.data_out_len = ( count * blockdev->blksize );
+	return ata_command ( ata, &command );
 }
 
 /**
 
 	/* Issue IDENTIFY */
 	memset ( &command, 0, sizeof ( command ) );
-	command.cb.device = ( ata->flags | ATA_DEV_OBSOLETE | ATA_DEV_LBA );
+	command.cb.count.native = 1; /* n/a according to spec, but at least
+				      * AoE vblade devices require it. */
+	command.cb.device = ( ata->device | ATA_DEV_OBSOLETE | ATA_DEV_LBA );
 	command.cb.cmd_stat = ATA_CMD_IDENTIFY;
-	command.data = virt_to_user ( &identity );
-	command.data_len = sizeof ( identity );
+	command.data_in = virt_to_user ( &identity );
+	command.data_in_len = sizeof ( identity );
 	if ( ( rc = ata_command ( ata, &command ) ) != 0 )
 		return rc;
 
 	/* Fill in block device parameters */
 	blockdev->blksize = ATA_SECTOR_SIZE;
 	if ( identity.supports_lba48 & cpu_to_le16 ( ATA_SUPPORTS_LBA48 ) ) {
-		ata->flags |= ATA_FL_LBA48;
+		ata->lba48 = 1;
 		blockdev->blocks = le64_to_cpu ( identity.lba48_sectors );
 	} else {
 		blockdev->blocks = le32_to_cpu ( identity.lba_sectors );

src/include/gpxe/ata.h

 		uint8_t low_prev;
 		uint8_t mid_prev;
 		uint8_t high_prev;
-		uint8_t pad[2];
+		uint16_t pad;
 #elif __BYTE_ORDER == __BIG_ENDIAN
-		uint8_t pad[2];
+		uint16_t pad;
 		uint8_t high_prev;
 		uint8_t mid_prev;
 		uint8_t low_prev;
 	uint8_t device;
 	/** Command/status register */
 	uint8_t cmd_stat;
-	/** Flags
-	 *
-	 * This field does not correspond to any ATA register.
-	 */
-	uint8_t flags;
+	/** LBA48 addressing flag */
+	int lba48;
 };
 
-/** LBA48 extended addressing */
-#define ATA_FL_LBA48 0x40
-
-/** Device 1 ("slave") */
-#define ATA_FL_SLAVE 0x10
-
-/** Write command */
-#define ATA_FL_WRITE 0x01
-
 /** Obsolete bits in the ATA device register */
 #define ATA_DEV_OBSOLETE 0xa0
 
 /** LBA flag in the ATA device register */
 #define ATA_DEV_LBA 0x40
 
+/** Slave ("device 1") flag in the ATA device register */
+#define ATA_DEV_SLAVE 0x10
+
+/** Master ("device 0") flag in the ATA device register */
+#define ATA_DEV_MASTER 0x00
+
 /** "Read sectors" command */
 #define ATA_CMD_READ 0x20
 
+/** "Read sectors (ext)" command */
+#define ATA_CMD_READ_EXT 0x24
+
 /** "Write sectors" command */
 #define ATA_CMD_WRITE 0x30
 
+/** "Write sectors (ext)" command */
+#define ATA_CMD_WRITE_EXT 0x34
+
 /** "Identify" command */
 #define ATA_CMD_IDENTIFY 0xec
 
-/** "Extended (LBA48)" command modifier
- *
- * This doesn't apply to all ATA commands, but it does for @c
- * ATA_CMD_READ and @c ATA_CMD_WRITE.
- */
-#define ATA_CMD_EXT 0x04
-
 /** An ATA command */
 struct ata_command {
 	/** ATA command block */
 	struct ata_cb cb;
-	/** Data buffer */
-	userptr_t data;
-	/** Data buffer length */
-	size_t data_len;
+	/** Data-out buffer (may be NULL) */
+	userptr_t data_out;
+	/** Data-out buffer length
+	 *
+	 * Must be zero if @c data_out is NULL
+	 */
+	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;
 };
 
 /**
 struct ata_device {
 	/** Block device interface */
 	struct block_device blockdev;
-	/** Flags */
-	int flags;
+	/** Device number
+	 *
+	 * Must be ATA_DEV_MASTER or ATA_DEV_SLAVE.
+	 */
+	int device;
+	/** LBA48 extended addressing */
+	int lba48;
 	/**
 	 * Issue ATA command
 	 *