device-dax: Add /sys/class/dax backwards compatibility

On the expectation that some environments may not upgrade libdaxctl
(userspace component that depends on the /sys/class/dax hierarchy),
provide a default / legacy dax_pmem_compat driver. The dax_pmem_compat
driver implements the original /sys/class/dax sysfs layout rather than
/sys/bus/dax. When userspace is upgraded it can blacklist this module
and switch to the dax_pmem driver going forward.

CONFIG_DEV_DAX_PMEM_COMPAT and supporting code will be deleted according
to the dax_pmem entry in Documentation/ABI/obsolete/.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 40 insertions, 0 deletions

diff --git a/drivers/dax/pmem/pmem.c b/drivers/dax/pmem/pmem.c
new file mode 100644
index 000000000000..0ae4238a0ef8
--- /dev/null
+++ b/drivers/dax/pmem/pmem.c
@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright(c) 2016 - 2018 Intel Corporation. All rights reserved. */
+#include <linux/percpu-refcount.h>
+#include <linux/memremap.h>
+#include <linux/module.h>
+#include <linux/pfn_t.h>
+#include <linux/nd.h>
+#include "../bus.h"
+
+static int dax_pmem_probe(struct device *dev)
+{
+	return PTR_ERR_OR_ZERO(__dax_pmem_probe(dev, DEV_DAX_BUS));
+}
+
+static struct nd_device_driver dax_pmem_driver = {
+	.probe = dax_pmem_probe,
+	.drv = {
+		.name = "dax_pmem",
+	},
+	.type = ND_DRIVER_DAX_PMEM,
+};
+
+static int __init dax_pmem_init(void)
+{
+	return nd_driver_register(&dax_pmem_driver);
+}
+module_init(dax_pmem_init);
+
+static void __exit dax_pmem_exit(void)
+{
+	driver_unregister(&dax_pmem_driver.drv);
+}
+module_exit(dax_pmem_exit);
+
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Intel Corporation");
+#if !IS_ENABLED(CONFIG_DEV_DAX_PMEM_COMPAT)
+/* For compat builds, don't load this module by default */
+MODULE_ALIAS_ND_DEVICE(ND_DEVICE_DAX_PMEM);
+#endif