libnvdimm/region: Fix nvdimm_has_flush() to handle ND_REGION_ASYNC

In case a platform doesn't provide explicit flush-hints but provides an
explicit flush callback via ND_REGION_ASYNC region flag, then
nvdimm_has_flush() still returns '0' indicating that writes do not
require flushing. This happens on PPC64 with patch at [1] applied, where
'deep_flush' of a region was denied even though an explicit flush
function was provided.

Fix this by adding a condition to nvdimm_has_flush() to test for the
ND_REGION_ASYNC flag on the region and see if a 'region->flush' callback
is assigned.

Link: http://lore.kernel.org/r/161703936121.36.7260632399582101498.stgit@e1fbed493c87 [1]
Fixes: c5d4355 ("libnvdimm: nd_region flush callback support")
Reported-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Link: https://lore.kernel.org/r/20210402092555.208590-1-vaibhav@linux.ibm.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/nvdimm/region_devs.c

@@ -1246,6 +1246,11 @@ int nvdimm_has_flush(struct nd_region *nd_region)
 			|| !IS_ENABLED(CONFIG_ARCH_HAS_PMEM_API))
 		return -ENXIO;
 
+	/* Test if an explicit flush function is defined */
+	if (test_bit(ND_REGION_ASYNC, &nd_region->flags) && nd_region->flush)
+		return 1;
+
+	/* Test if any flush hints for the region are available */
 	for (i = 0; i < nd_region->ndr_mappings; i++) {
 		struct nd_mapping *nd_mapping = &nd_region->mapping[i];
 		struct nvdimm *nvdimm = nd_mapping->nvdimm;
@@ -1256,8 +1261,8 @@ int nvdimm_has_flush(struct nd_region *nd_region)
 	}
 
 	/*
-	 * The platform defines dimm devices without hints, assume
-	 * platform persistence mechanism like ADR
+	 * The platform defines dimm devices without hints nor explicit flush,
+	 * assume platform persistence mechanism like ADR
 	 */
 	return 0;
 }