dax, dm: allow device-mapper to operate without dax support

Change device-mapper's DAX dependency to require the presence of at
least one DAX_DRIVER. This allows device-mapper to be built without
bringing the DAX core along which is especially wasteful when there are
no DAX drivers, like BLK_DEV_PMEM, configured.

Cc: Alasdair Kergon <agk@redhat.com>
Reported-by: Bart Van Assche <Bart.VanAssche@wdc.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 24 insertions, 6 deletions

diff --git a/include/linux/dax.h b/include/linux/dax.h
index ae27a7efe7ab..f9eb22ad341e 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -26,16 +26,39 @@ extern struct attribute_group dax_attribute_group;
 
 #if IS_ENABLED(CONFIG_DAX)
 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);
+void kill_dax(struct dax_device *dax_dev);
+void dax_write_cache(struct dax_device *dax_dev, bool wc);
+bool dax_write_cache_enabled(struct dax_device *dax_dev);
 #else
 static inline struct dax_device *dax_get_by_host(const char *host)
 {
 	return NULL;
 }
-
+static inline struct dax_device *alloc_dax(void *private, const char *host,
+		const struct dax_operations *ops)
+{
+	/*
+	 * Callers should check IS_ENABLED(CONFIG_DAX) to know if this
+	 * NULL is an error or expected.
+	 */
+	return NULL;
+}
 static inline void put_dax(struct dax_device *dax_dev)
 {
 }
+static inline void kill_dax(struct dax_device *dax_dev)
+{
+}
+static inline void dax_write_cache(struct dax_device *dax_dev, bool wc)
+{
+}
+static inline bool dax_write_cache_enabled(struct dax_device *dax_dev)
+{
+	return false;
+}
 #endif
 
 struct writeback_control;
@@ -89,18 +112,13 @@ static inline int dax_writeback_mapping_range(struct address_space *mapping,
 
 int dax_read_lock(void);
 void dax_read_unlock(int id);
-struct dax_device *alloc_dax(void *private, const char *host,
-		const struct dax_operations *ops);
 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);
 long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 		void **kaddr, pfn_t *pfn);
 size_t dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
 		size_t bytes, struct iov_iter *i);
 void dax_flush(struct dax_device *dax_dev, void *addr, size_t size);
-void dax_write_cache(struct dax_device *dax_dev, bool wc);
-bool dax_write_cache_enabled(struct dax_device *dax_dev);
 
 ssize_t dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
 		const struct iomap_ops *ops);