block, dax: move "select DAX" from BLOCK to FS_DAX

For configurations that do not enable DAX filesystems or drivers, do not
require the DAX core to be built.

Given that the 'direct_access' method has been removed from
'block_device_operations', we can also go ahead and remove the
block-related dax helper functions from fs/block_dev.c to
drivers/dax/super.c. This keeps dax details out of the block layer and
lets the DAX core be built as a module in the FS_DAX=n case.

Filesystems need to include dax.h to call bdev_dax_supported().

Cc: linux-xfs@vger.kernel.org
Cc: Jens Axboe <axboe@kernel.dk>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

2 files changed, 28 insertions, 4 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 848f87eb1905..e4d9899755a7 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1940,8 +1940,6 @@ extern int __blkdev_driver_ioctl(struct block_device *, fmode_t, unsigned int,
 extern int bdev_read_page(struct block_device *, sector_t, struct page *);
 extern int bdev_write_page(struct block_device *, sector_t, struct page *,
 						struct writeback_control *);
-extern int bdev_dax_supported(struct super_block *, int);
-int bdev_dax_pgoff(struct block_device *, sector_t, size_t, pgoff_t *pgoff);
 #else /* CONFIG_BLOCK */
 
 struct block_device;
diff --git a/include/linux/dax.h b/include/linux/dax.h
index d3158e74a59e..7fdf1d710042 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -18,12 +18,38 @@ struct dax_operations {
 			void **, pfn_t *);
 };
 
+int bdev_dax_pgoff(struct block_device *, sector_t, size_t, pgoff_t *pgoff);
+#if IS_ENABLED(CONFIG_FS_DAX)
+int __bdev_dax_supported(struct super_block *sb, int blocksize);
+static inline int bdev_dax_supported(struct super_block *sb, int blocksize)
+{
+	return __bdev_dax_supported(sb, blocksize);
+}
+#else
+static inline int bdev_dax_supported(struct super_block *sb, int blocksize)
+{
+	return -EOPNOTSUPP;
+}
+#endif
+
+#if IS_ENABLED(CONFIG_DAX)
+struct dax_device *dax_get_by_host(const char *host);
+void put_dax(struct dax_device *dax_dev);
+#else
+static inline struct dax_device *dax_get_by_host(const char *host)
+{
+	return NULL;
+}
+
+static inline void put_dax(struct dax_device *dax_dev)
+{
+}
+#endif
+
 int dax_read_lock(void);
 void dax_read_unlock(int id);
-struct dax_device *dax_get_by_host(const char *host);
 struct dax_device *alloc_dax(void *private, const char *host,
 		const struct dax_operations *ops);
-void put_dax(struct dax_device *dax_dev);
 bool dax_alive(struct dax_device *dax_dev);
 void kill_dax(struct dax_device *dax_dev);
 void *dax_get_private(struct dax_device *dax_dev);