dax: stub out dax_supported for !CONFIG_FS_DAX

dax_supported calls into ->dax_supported which checks for fsdax support.
Don't bother building it for !CONFIG_FS_DAX as it will always return
false.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20210826135510.6293-8-hch@lst.deSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dax/super.c

@@ -201,6 +201,24 @@ bool generic_fsdax_supported(struct dax_device *dax_dev,
 	return true;
 }
 EXPORT_SYMBOL_GPL(generic_fsdax_supported);
+
+bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
+		int blocksize, sector_t start, sector_t len)
+{
+	bool ret = false;
+	int id;
+
+	if (!dax_dev)
+		return false;
+
+	id = dax_read_lock();
+	if (dax_alive(dax_dev) && dax_dev->ops->dax_supported)
+		ret = dax_dev->ops->dax_supported(dax_dev, bdev, blocksize,
+						  start, len);
+	dax_read_unlock(id);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(dax_supported);
 #endif /* CONFIG_FS_DAX */
 
 /**
@@ -350,24 +368,6 @@ long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 }
 EXPORT_SYMBOL_GPL(dax_direct_access);
 
-bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
-		int blocksize, sector_t start, sector_t len)
-{
-	bool ret = false;
-	int id;
-
-	if (!dax_dev)
-		return false;
-
-	id = dax_read_lock();
-	if (dax_alive(dax_dev) && dax_dev->ops->dax_supported)
-		ret = dax_dev->ops->dax_supported(dax_dev, bdev, blocksize,
-						  start, len);
-	dax_read_unlock(id);
-	return ret;
-}
-EXPORT_SYMBOL_GPL(dax_supported);
-
 size_t dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
 		size_t bytes, struct iov_iter *i)
 {

include/linux/dax.h

@@ -57,8 +57,6 @@ static inline void set_dax_synchronous(struct dax_device *dax_dev)
 {
 	__set_dax_synchronous(dax_dev);
 }
-bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
-		int blocksize, sector_t start, sector_t len);
 /*
  * Check if given mapping is supported by the file / underlying device.
  */
@@ -101,12 +99,6 @@ static inline bool dax_synchronous(struct dax_device *dax_dev)
 static inline void set_dax_synchronous(struct dax_device *dax_dev)
 {
 }
-static inline bool dax_supported(struct dax_device *dax_dev,
-		struct block_device *bdev, int blocksize, sector_t start,
-		sector_t len)
-{
-	return false;
-}
 static inline bool daxdev_mapping_supported(struct vm_area_struct *vma,
 				struct dax_device *dax_dev)
 {
@@ -127,6 +119,9 @@ bool generic_fsdax_supported(struct dax_device *dax_dev,
 		struct block_device *bdev, int blocksize, sector_t start,
 		sector_t sectors);
 
+bool dax_supported(struct dax_device *dax_dev, struct block_device *bdev,
+		int blocksize, sector_t start, sector_t len);
+
 static inline void fs_put_dax(struct dax_device *dax_dev)
 {
 	put_dax(dax_dev);
@@ -149,6 +144,13 @@ static inline bool bdev_dax_supported(struct block_device *bdev,
 
 #define generic_fsdax_supported		NULL
 
+static inline bool dax_supported(struct dax_device *dax_dev,
+		struct block_device *bdev, int blocksize, sector_t start,
+		sector_t len)
+{
+	return false;
+}
+
 static inline void fs_put_dax(struct dax_device *dax_dev)
 {
 }