[block] Allow SAN retry count to be reconfigured

Allow the SAN retry count to be configured via the ${san-retry}
setting, defaulting to the current value of 10 retries if not
specified.

Note that setting a retry count of zero is inadvisable, since iSCSI
targets in particular will often report spurious errors such as "power
on occurred" for the first few commands.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/dhcp.h

@@ -433,6 +433,13 @@ struct dhcp_netdev_desc {
 /** Use cached network settings (obsolete; do not reuse this value) */
 #define DHCP_EB_USE_CACHED DHCP_ENCAP_OPT ( DHCP_EB_ENCAP, 0xb2 )
 
+/** SAN retry count
+ *
+ * This is the maximum number of times that SAN operations will be
+ * retried.
+ */
+#define DHCP_EB_SAN_RETRY DHCP_ENCAP_OPT ( DHCP_EB_ENCAP, 0xbb )
+
 /** SAN drive number
  *
  * This is the drive number for a SAN-hooked drive.  For BIOS, 0x80 is

src/include/ipxe/sanboot.h

@@ -71,6 +71,8 @@ struct san_device {
 	/** Driver private data */
 	void *priv;
 
+	/** Number of paths */
+	unsigned int paths;
 	/** Current active path */
 	struct san_path *active;
 	/** List of opened SAN paths */