When unloading the module, one gets
[  548.188594] ------------[ cut here ]------------
[  548.188596] Device 'cmm0' does not have a release() function, it is broken and must be fixed. See Documentation/kobject.txt.
[  548.188622] WARNING: CPU: 0 PID: 19308 at drivers/base/core.c:1244 .device_release+0xcc/0xf0
...

We only have on static fake device. There is nothing to do when
releasing the device (via cmm_exit).

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <redacted>
Cc: Konstantin Khlebnikov <redacted>
Cc: Allison Randal <redacted>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Thomas Gleixner <redacted>
Cc: Arun KS <redacted>
Signed-off-by: David Hildenbrand <redacted>
---
 arch/powerpc/platforms/pseries/cmm.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/powerpc/platforms/pseries/cmm.c b/arch/powerpc/platforms/pseries/cmm.c
index b33251d75927..572651a5c87b 100644
--- a/arch/powerpc/platforms/pseries/cmm.c
+++ b/arch/powerpc/platforms/pseries/cmm.c

@@ -411,6 +411,10 @@ static struct bus_type cmm_subsys = {
 	.dev_name = "cmm",
 };
 
+static void cmm_release_device(struct device *dev)
+{
+}
+
 /**
  * cmm_sysfs_register - Register with sysfs
  *

@@ -426,6 +430,7 @@ static int cmm_sysfs_register(struct device *dev)
 
 	dev->id = 0;
 	dev->bus = &cmm_subsys;
+	dev->release = cmm_release_device;
 
 	if ((rc = device_register(dev)))
 		goto subsys_unregister;

-- 
2.21.0