[PATCH] libnvdimm, pmem: clarify the write+clear_poison+write flow

[Dan Williams]

The ACPI specification does not specify the state of data after a clear
poison operation.  Potential future libnvdimm bus implementations for
other architectures also might not specify or disagree on the state of
data after clear poison.  Clarify why we write twice.

Reported-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
Reported-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 drivers/nvdimm/pmem.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
index c6befaa9c708..d9a0dbc2d023 100644
--- a/drivers/nvdimm/pmem.c
+++ b/drivers/nvdimm/pmem.c
@@ -86,6 +86,20 @@ static int pmem_do_bvec(struct pmem_device *pmem, struct page *page,
 			flush_dcache_page(page);
 		}
 	} else {
+		/*
+		 * Note that we write the data both before and after
+		 * clearing poison.  The write before clear poison
+		 * handles situations where the latest written data is
+		 * preserved and the clear poison operation simply marks
+		 * the address range as valid without changing the data.
+		 * In this case application software can assume that an
+		 * interrupted write will either return the new good
+		 * data or an error.
+		 *
+		 * However, if pmem_clear_poison() leaves the data in an
+		 * indeterminate state we need to perform the write
+		 * after clear poison.
+		 */
 		flush_dcache_page(page);
 		memcpy_to_pmem(pmem_addr, mem + off, len);
 		if (unlikely(bad_pmem)) {