null_blk: remove explicit 'select FAULT_INJECTION'

Selecting FAULT_INJECTION causes a Kconfig warning when CONFIG_DEBUG_KERNEL
is not set:

warning: (BLK_DEV_NULL_BLK && DRM_I915_SELFTEST) selects FAULT_INJECTION which has unmet direct dependencies (DEBUG_KERNEL)

The other drivers that use FAULT_INJECTION tend to have a separate
Kconfig symbol for turning on that feature, so let's do the same
thing here. This may add a bit more complexity than we like, but
it avoids the warning and is more consistent with the rest of the
kernel.

Fixes: 93b5704 ("null_blk: add option for managing IO timeouts")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/Kconfig

@@ -19,7 +19,10 @@ if BLK_DEV
 config BLK_DEV_NULL_BLK
 	tristate "Null test block driver"
 	select CONFIGFS_FS
-	select FAULT_INJECTION
+
+config BLK_DEV_NULL_BLK_FAULT_INJECTION
+	bool "Support fault injection for Null test block driver"
+	depends on BLK_DEV_NULL_BLK && FAULT_INJECTION
 
 config BLK_DEV_FD
 	tristate "Normal floppy disk support"

drivers/block/null_blk.c

@@ -27,7 +27,9 @@
 #define TICKS_PER_SEC		50ULL
 #define TIMER_INTERVAL		(NSEC_PER_SEC / TICKS_PER_SEC)
 
+#ifdef CONFIG_BLK_DEV_NULL_BLK_FAULT_INJECTION
 static DECLARE_FAULT_ATTR(null_timeout_attr);
+#endif
 
 static inline u64 mb_per_tick(int mbps)
 {
@@ -165,8 +167,10 @@ static int g_home_node = NUMA_NO_NODE;
 module_param_named(home_node, g_home_node, int, S_IRUGO);
 MODULE_PARM_DESC(home_node, "Home node for the device");
 
+#ifdef CONFIG_BLK_DEV_NULL_BLK_FAULT_INJECTION
 static char g_timeout_str[80];
 module_param_string(timeout, g_timeout_str, sizeof(g_timeout_str), S_IRUGO);
+#endif
 
 static int g_queue_mode = NULL_Q_MQ;
 
@@ -1372,8 +1376,10 @@ static int null_rq_prep_fn(struct request_queue *q, struct request *req)
 
 static bool should_timeout_request(struct request *rq)
 {
+#ifdef CONFIG_BLK_DEV_NULL_BLK_FAULT_INJECTION
 	if (g_timeout_str[0])
 		return should_fail(&null_timeout_attr, 1);
+#endif
 
 	return false;
 }
@@ -1655,13 +1661,15 @@ static void null_validate_conf(struct nullb_device *dev)
 
 static bool null_setup_fault(void)
 {
+#ifdef CONFIG_BLK_DEV_NULL_BLK_FAULT_INJECTION
 	if (!g_timeout_str[0])
 		return true;
 
 	if (!setup_fault_attr(&null_timeout_attr, g_timeout_str))
 		return false;
 
 	null_timeout_attr.verbose = 0;
+#endif
 	return true;
 }