[int13] Allow default drive to be specified via "san-drive" setting

The DHCP option 175.189 has been defined (by us) since 2006 as
containing the drive number to be used for a SAN boot, but has never
been automatically used as such by iPXE.

Use this option (if specified) to override the default SAN drive
number.

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

src/arch/x86/include/ipxe/bios_sanboot.h

 #define SANBOOT_PREFIX_pcbios __pcbios_
 #endif
 
-/**
- * Get default SAN drive number
- *
- * @ret drive		Default drive number
- */
-static inline __always_inline unsigned int
-SANBOOT_INLINE ( pcbios, san_default_drive ) ( void ) {
-	/* Default to booting from first hard disk */
-	return 0x80;
-}
-
 #endif /* _IPXE_BIOS_SANBOOT_H */

src/arch/x86/interface/pcbios/int13.c

 #include <ipxe/pci.h>
 #include <ipxe/iso9660.h>
 #include <ipxe/eltorito.h>
+#include <ipxe/dhcp.h>
+#include <ipxe/settings.h>
 #include <realmode.h>
 #include <bios.h>
 #include <biosint.h>
 	return 0;
 }
 
-PROVIDE_SANBOOT_INLINE ( pcbios, san_default_drive );
+/** The "san-drive" setting */
+const struct setting san_drive_setting __setting ( SETTING_SANBOOT_EXTRA,
+						   san-drive ) = {
+	.name = "san-drive",
+	.description = "SAN drive number",
+	.tag = DHCP_EB_SAN_DRIVE,
+	.type = &setting_type_uint8,
+};
+
+/**
+ * Get default SAN drive number
+ *
+ * @ret drive		Default drive number
+ */
+static unsigned int int13_default_drive ( void ) {
+	unsigned long drive;
+
+	/* Use "san-drive" setting, if specified */
+	if ( fetch_uint_setting ( NULL, &san_drive_setting, &drive ) >= 0 )
+		return drive;
+
+	/* Otherwise, default to booting from first hard disk */
+	return 0x80;
+}
+
+PROVIDE_SANBOOT ( pcbios, san_default_drive, int13_default_drive );
 PROVIDE_SANBOOT ( pcbios, san_hook, int13_hook );
 PROVIDE_SANBOOT ( pcbios, san_unhook, int13_unhook );
 PROVIDE_SANBOOT ( pcbios, san_boot, int13_boot );

src/include/ipxe/dhcp.h

 /** Use cached network settings (obsolete; do not reuse this value) */
 #define DHCP_EB_USE_CACHED DHCP_ENCAP_OPT ( DHCP_EB_ENCAP, 0xb2 )
 
-/** BIOS drive number
+/** SAN drive number
  *
- * This is the drive number for a drive emulated via INT 13.  0x80 is
+ * This is the drive number for a SAN-hooked drive.  For BIOS, 0x80 is
  * the first hard disk, 0x81 is the second hard disk, etc.
  */
-#define DHCP_EB_BIOS_DRIVE DHCP_ENCAP_OPT ( DHCP_EB_ENCAP, 0xbd )
+#define DHCP_EB_SAN_DRIVE DHCP_ENCAP_OPT ( DHCP_EB_ENCAP, 0xbd )
 
 /** Username
  *