[pxe] Create pxe_[de]activate() wrapper functions

Merge the pxe_set_netdev()+pxe_[un]hook_int1a() pattern into a single
pxe_[de]activate() call.

src/arch/i386/image/pxe_image.c

  * @ret rc		Return status code
  */
 static int pxe_exec ( struct image *image ) {
+	struct net_device *netdev;
 	int rc;
 
-	/* Ensure that PXE stack is ready to use */
-	pxe_hook_int1a();
-
 	/* Arbitrarily pick the most recently opened network device */
-	pxe_set_netdev ( last_opened_netdev() );
-
-	/* Many things will break if pxe_netdev is NULL */
-	if ( ! pxe_netdev ) {
+	if ( ( netdev = last_opened_netdev() ) == NULL ) {
 		DBGC ( image, "IMAGE %p could not locate PXE net device\n",
 		       image );
 		return -ENODEV;
 	}
 
+	/* Activate PXE */
+	pxe_activate ( netdev );
+
 	/* Start PXE NBP */
 	rc = pxe_start_nbp();
 
 	/* Deactivate PXE */
-	pxe_set_netdev ( NULL );
-	pxe_unhook_int1a();
+	pxe_deactivate();
 
 	return rc;
 }

src/arch/i386/include/pxe_call.h

 #include <pxe_api.h>
 #include <realmode.h>
 
+struct net_device;
+
 /** PXE load address segment */
 #define PXE_LOAD_SEGMENT 0
 
 extern struct s_PXENV __text16 ( pxenv );
 #define pxenv __use_text16 ( pxenv )
 
-extern void pxe_hook_int1a ( void );
-extern int pxe_unhook_int1a ( void );
+extern void pxe_activate ( struct net_device *netdev );
+extern int pxe_deactivate ( void );
 extern int pxe_start_nbp ( void );
 extern __asmcall void pxe_api_call ( struct i386_all_regs *ix86 );
 

src/arch/i386/interface/pxe/pxe_call.c

 /** INT 1A handler */
 extern void pxe_int_1a ( void );
 
+/** INT 1A hooked flag */
+static int int_1a_hooked = 0;
+
 /** A function pointer to hold any PXE API call
  *
  * Used by pxe_api_call() to avoid large swathes of duplicated code.
 	ix86->regs.ax = ret;
 }
 
-/**
- * Hook INT 1A for PXE
- *
- */
-void pxe_hook_int1a ( void ) {
-	hook_bios_interrupt ( 0x1a, ( unsigned int ) pxe_int_1a,
-			      &pxe_int_1a_vector );
-}
-
-/**
- * Unhook INT 1A for PXE
- *
- * @ret rc		Return status code
- */
-int pxe_unhook_int1a ( void ) {
-	return unhook_bios_interrupt ( 0x1a, ( unsigned int ) pxe_int_1a,
-				       &pxe_int_1a_vector );
-}
-
 /**
  * Calculate byte checksum as used by PXE
  *
 	.initialise = pxe_init_structures,
 };
 
+/**
+ * Activate PXE stack
+ *
+ * @v netdev		Net device to use as PXE net device
+ */
+void pxe_activate ( struct net_device *netdev ) {
+
+	/* Ensure INT 1A is hooked */
+	if ( ! int_1a_hooked ) {
+		hook_bios_interrupt ( 0x1a, ( unsigned int ) pxe_int_1a,
+				      &pxe_int_1a_vector );
+		int_1a_hooked = 1;
+	}
+
+	/* Set PXE network device */
+	pxe_set_netdev ( netdev );
+}
+
+/**
+ * Deactivate PXE stack
+ *
+ * @ret rc		Return status code
+ */
+int pxe_deactivate ( void ) {
+	int rc;
+
+	/* Clear PXE network device */
+	pxe_set_netdev ( NULL );
+
+	/* Ensure INT 1A is unhooked, if possible */
+	if ( int_1a_hooked ) {
+		if ( ( rc = unhook_bios_interrupt ( 0x1a,
+						    (unsigned int) pxe_int_1a,
+						    &pxe_int_1a_vector ))!= 0){
+			DBG ( "Could not unhook INT 1A: %s\n",
+			      strerror ( rc ) );
+			return rc;
+		}
+		int_1a_hooked = 0;
+	}
+
+	return 0;
+}
+
 /**
  * Start PXE NBP at 0000:7c00
  *

src/arch/i386/interface/pxe/pxe_preboot.c

 	}
 	DBG ( " using netdev %s", netdev->name );
 
-	/* Save as PXE net device */
-	pxe_set_netdev ( netdev );
-
-	/* Hook INT 1A */
-	pxe_hook_int1a();
+	/* Activate PXE */
+	pxe_activate ( netdev );
 
 	start_undi->Status = PXENV_STATUS_SUCCESS;
 	return PXENV_EXIT_SUCCESS;
 PXENV_EXIT_t pxenv_stop_undi ( struct s_PXENV_STOP_UNDI *stop_undi ) {
 	DBG ( "PXENV_STOP_UNDI" );
 
-	/* Unhook INT 1A */
-	pxe_unhook_int1a();
-
-	/* Clear PXE net device */
-	pxe_set_netdev ( NULL );
+	/* Deactivate PXE */
+	pxe_deactivate();
 
 	/* Prepare for unload */
 	shutdown ( SHUTDOWN_BOOT );