[PATCH 2/2] drivers/dax/kmem: give a warning if CONFIG_DEV_DAX_PMEM_COMPAT is enabled

[Jia He]

commit c221c0b0308f ("device-dax: "Hotplug" persistent memory for use
like normal RAM") helps to add persistent memory as normal RAM blocks.
But this driver doesn't work if CONFIG_DEV_DAX_PMEM_COMPAT is enabled.

Here is the debugging call trace when CONFIG_DEV_DAX_PMEM_COMPAT is
enabled.
[    4.443730]  devm_memremap_pages+0x4b9/0x540
[    4.443733]  dev_dax_probe+0x112/0x220 [device_dax]
[    4.443735]  dax_pmem_compat_probe+0x58/0x92 [dax_pmem_compat]
[    4.443737]  nvdimm_bus_probe+0x6b/0x150
[    4.443739]  really_probe+0xf5/0x3d0
[    4.443740]  driver_probe_device+0x11b/0x130
[    4.443741]  device_driver_attach+0x58/0x60
[    4.443742]  __driver_attach+0xa3/0x140

Then the dax0.0 device will be registered as "nd" bus instead of
"dax" bus. This causes the error as follows:
root@ubuntu:~# echo dax0.0 > /sys/bus/dax/drivers/device_dax/unbind
-bash: echo: write error: No such device

This gives a warning to notify the user.

Signed-off-by: Jia He <justin.he@xxxxxxx>
---
 drivers/dax/kmem.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
index ad62d551d94e..b77f0e880598 100644
--- a/drivers/dax/kmem.c
+++ b/drivers/dax/kmem.c
@@ -93,6 +93,11 @@ static struct dax_device_driver device_dax_kmem_driver = {
 
 static int __init dax_kmem_init(void)
 {
+	if (IS_ENABLED(CONFIG_DEV_DAX_PMEM_COMPAT)) {
+		pr_warn("CONFIG_DEV_DAX_PMEM_COMPAT is not compatible\n");
+		pr_warn("kmem dax driver might not be workable\n");
+	}
+
 	return dax_driver_register(&device_dax_kmem_driver);
 }
 
-- 
2.17.1