[PATCH V3 05/21] dax: Add dax_set_ops() for setting dax_operations at bind time

[John Groves <John@Groves.net>]

From: John Groves <John@Groves.net>

The dax_device is created (in the non-pmem case) at hmem probe time via
devm_create_dev_dax(), before we know which driver (device_dax,
fsdev_dax, or kmem) will bind - by calling alloc_dax() with NULL ops,
drivers (i.e. fsdev_dax) that need specific dax_operations must set
them later.

Add dax_set_ops() exported function so fsdev_dax can set its ops at
probe time and clear them on remove. device_dax doesn't need ops since
it uses the mmap fault path directly.

Use cmpxchg() to atomically set ops only if currently NULL, returning
-EBUSY if ops are already set. This prevents accidental double-binding.
Clearing ops (NULL) always succeeds.

Signed-off-by: John Groves <john@groves.net>
---
 drivers/dax/fsdev.c | 12 ++++++++++++
 drivers/dax/super.c | 38 +++++++++++++++++++++++++++++++++++++-
 include/linux/dax.h |  1 +
 3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/drivers/dax/fsdev.c b/drivers/dax/fsdev.c
index 9e2f83aa2584..3f4f593896e3 100644
--- a/drivers/dax/fsdev.c
+++ b/drivers/dax/fsdev.c
@@ -330,12 +330,24 @@ static int fsdev_dax_probe(struct dev_dax *dev_dax)
 	if (rc)
 		return rc;
 
+	/* Set the dax operations for fs-dax access path */
+	rc = dax_set_ops(dax_dev, &dev_dax_ops);
+	if (rc)
+		return rc;
+
 	run_dax(dax_dev);
 	return devm_add_action_or_reset(dev, fsdev_kill, dev_dax);
 }
 
+static void fsdev_dax_remove(struct dev_dax *dev_dax)
+{
+	/* Clear ops on unbind so they aren't used with a different driver */
+	dax_set_ops(dev_dax->dax_dev, NULL);
+}
+
 static struct dax_device_driver fsdev_dax_driver = {
 	.probe = fsdev_dax_probe,
+	.remove = fsdev_dax_remove,
 	.type = DAXDRV_FSDEV_TYPE,
 };
 
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index c00b9dff4a06..ba0b4cd18a77 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -157,6 +157,9 @@ long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 	if (!dax_alive(dax_dev))
 		return -ENXIO;
 
+	if (!dax_dev->ops)
+		return -EOPNOTSUPP;
+
 	if (nr_pages < 0)
 		return -EINVAL;
 
@@ -207,6 +210,10 @@ int dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
 
 	if (!dax_alive(dax_dev))
 		return -ENXIO;
+
+	if (!dax_dev->ops)
+		return -EOPNOTSUPP;
+
 	/*
 	 * There are no callers that want to zero more than one page as of now.
 	 * Once users are there, this check can be removed after the
@@ -223,7 +230,7 @@ EXPORT_SYMBOL_GPL(dax_zero_page_range);
 size_t dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
 		void *addr, size_t bytes, struct iov_iter *iter)
 {
-	if (!dax_dev->ops->recovery_write)
+	if (!dax_dev->ops || !dax_dev->ops->recovery_write)
 		return 0;
 	return dax_dev->ops->recovery_write(dax_dev, pgoff, addr, bytes, iter);
 }
@@ -307,6 +314,35 @@ void set_dax_nomc(struct dax_device *dax_dev)
 }
 EXPORT_SYMBOL_GPL(set_dax_nomc);
 
+/**
+ * dax_set_ops - set the dax_operations for a dax_device
+ * @dax_dev: the dax_device to configure
+ * @ops: the operations to set (may be NULL to clear)
+ *
+ * This allows drivers to set the dax_operations after the dax_device
+ * has been allocated. This is needed when the device is created before
+ * the driver that needs specific ops is bound (e.g., fsdev_dax binding
+ * to a dev_dax created by hmem).
+ *
+ * When setting non-NULL ops, fails if ops are already set (returns -EBUSY).
+ * When clearing ops (NULL), always succeeds.
+ *
+ * Return: 0 on success, -EBUSY if ops already set
+ */
+int dax_set_ops(struct dax_device *dax_dev, const struct dax_operations *ops)
+{
+	if (ops) {
+		/* Setting ops: fail if already set */
+		if (cmpxchg(&dax_dev->ops, NULL, ops) != NULL)
+			return -EBUSY;
+	} else {
+		/* Clearing ops: always allowed */
+		dax_dev->ops = NULL;
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(dax_set_ops);
+
 bool dax_alive(struct dax_device *dax_dev)
 {
 	lockdep_assert_held(&dax_srcu);
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 74e098010016..3fcd8562b72b 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -246,6 +246,7 @@ static inline void dax_break_layout_final(struct inode *inode)
 
 bool dax_alive(struct dax_device *dax_dev);
 void *dax_get_private(struct dax_device *dax_dev);
+int dax_set_ops(struct dax_device *dax_dev, const struct dax_operations *ops);
 long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
 		enum dax_access_mode mode, void **kaddr, unsigned long *pfn);
 size_t dax_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff, void *addr,
-- 
2.49.0