cdrom: don't attempt to fiddle with cdo->capability

We can't modify cdo->capability as it is defined as a const.
Change the modification hack to just WARN_ON_ONCE() if we hit
any of the invalid combinations.

This fixes a regression for pcd, which doesn't work after the
constify patch.

Fixes: 853fe1bf ("cdrom: Make device operations read-only")
Tested-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/cdrom/cdrom.c

@@ -410,10 +410,10 @@ static int cdrom_get_disc_info(struct cdrom_device_info *cdi,
  * hack to have the capability flags defined const, while we can still
  * change it here without gcc complaining at every line.
  */
-#define ENSURE(call, bits)			\
+#define ENSURE(cdo, call, bits)					\
 do {								\
 	if (cdo->call == NULL)					\
-		*change_capability &= ~(bits);	\
+		WARN_ON_ONCE((cdo)->capability & (bits));	\
 } while (0)
 
 /*
@@ -589,7 +589,6 @@ int register_cdrom(struct cdrom_device_info *cdi)
 {
 	static char banner_printed;
 	const struct cdrom_device_ops *cdo = cdi->ops;
-	int *change_capability = (int *)&cdo->capability; /* hack */
 
 	cd_dbg(CD_OPEN, "entering register_cdrom\n");
 
@@ -601,16 +600,16 @@ int register_cdrom(struct cdrom_device_info *cdi)
 		cdrom_sysctl_register();
 	}
 
-	ENSURE(drive_status, CDC_DRIVE_STATUS);
+	ENSURE(cdo, drive_status, CDC_DRIVE_STATUS);
 	if (cdo->check_events == NULL && cdo->media_changed == NULL)
-		*change_capability = ~(CDC_MEDIA_CHANGED | CDC_SELECT_DISC);
-	ENSURE(tray_move, CDC_CLOSE_TRAY | CDC_OPEN_TRAY);
-	ENSURE(lock_door, CDC_LOCK);
-	ENSURE(select_speed, CDC_SELECT_SPEED);
-	ENSURE(get_last_session, CDC_MULTI_SESSION);
-	ENSURE(get_mcn, CDC_MCN);
-	ENSURE(reset, CDC_RESET);
-	ENSURE(generic_packet, CDC_GENERIC_PACKET);
+		WARN_ON_ONCE(cdo->capability & (CDC_MEDIA_CHANGED | CDC_SELECT_DISC));
+	ENSURE(cdo, tray_move, CDC_CLOSE_TRAY | CDC_OPEN_TRAY);
+	ENSURE(cdo, lock_door, CDC_LOCK);
+	ENSURE(cdo, select_speed, CDC_SELECT_SPEED);
+	ENSURE(cdo, get_last_session, CDC_MULTI_SESSION);
+	ENSURE(cdo, get_mcn, CDC_MCN);
+	ENSURE(cdo, reset, CDC_RESET);
+	ENSURE(cdo, generic_packet, CDC_GENERIC_PACKET);
 	cdi->mc_flags = 0;
 	cdi->options = CDO_USE_FFLAGS;