[PATCH v3] libnvdimm/region: Update nvdimm_has_flush() to handle explicit 'flush' callbacks

[Vaibhav Jain <vaibhav@linux.ibm.com>]

In case a platform doesn't provide explicit flush-hints but provides an
explicit flush callback, 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.

Similar problem is also seen with virtio-pmem where the 'deep_flush'
sysfs attribute is not visible as in absence of any registered nvdimm,
'nd_region->ndr_mappings == 0'.

Fix this by updating nvdimm_has_flush() adding a condition to
nvdimm_has_flush() to test if a 'region->flush' callback is
assigned. Also remove explicit test for 'nd_region->ndr_mapping' since
regions may need 'flush' without any explicit mappings as in case of
virtio-pmem.

References:
[1] "powerpc/papr_scm: Implement support for H_SCM_FLUSH hcall"
https://lore.kernel.org/linux-nvdimm/161703936121.36.7260632399582101498.stgit@e1fbed493c87

Cc: <stable@vger.kernel.org>
Fixes: c5d4355d10d4 ("libnvdimm: nd_region flush callback support")
Reported-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
---
Changelog:

v3:
* Removed the test for ND_REGION_SYNC to handle case where a
  synchronous region still wants to expose a deep-flush function.
  [ Aneesh ]
* Updated patch title and description from previous patch
  https://lore.kernel.org/linux-nvdimm/5e64778d-bf48-9f10-7d3d-5e530e5db590@linux.ibm.com

v2:
* Added the fixes tag and addressed the patch to stable tree [ Aneesh ]
* Updated patch description to address the virtio-pmem case.
* Removed test for 'nd_region->ndr_mappings' from beginning of
  nvdimm_has_flush() to handle the virtio-pmem case.
---
 drivers/nvdimm/region_devs.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
index ef23119db574..c4b17bdd527f 100644
--- a/drivers/nvdimm/region_devs.c
+++ b/drivers/nvdimm/region_devs.c
@@ -1234,11 +1234,15 @@ int nvdimm_has_flush(struct nd_region *nd_region)
 {
 	int i;
 
-	/* no nvdimm or pmem api == flushing capability unknown */
-	if (nd_region->ndr_mappings == 0
-			|| !IS_ENABLED(CONFIG_ARCH_HAS_PMEM_API))
+	/* no pmem api == flushing capability unknown */
+	if (!IS_ENABLED(CONFIG_ARCH_HAS_PMEM_API))
 		return -ENXIO;
 
+	/* Test if an explicit flush function is defined */
+	if (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;
@@ -1249,8 +1253,8 @@ 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;
 }
-- 
2.30.2
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