drivers/nvdimm/e820: turn off write cache by default

Commit Message

Roman Kagan Sept. 29, 2022, 7:32 a.m. UTC

When regular DRAM is registered for use as PMEM via "memmap" command
line parameter, there's no write cache in front of the backing store of
this PMEM (as there's no backing store itself), so there's no point
doing expensive cache flush on sync etc.

Mark the regions being registered with e820 as ND_REGION_PERSIST_CACHE
so that write cache is off by default for the respective DAX devices.
This also matches the assumed behavior of the flag
ND_REGION_PERSIST_CACHE:

  Platform ensures entire CPU store data path is flushed to pmem on
  system power loss.

for the only usecase where such regions actually kind of persist the
data -- across kexec.

Signed-off-by: Roman Kagan <[e-mail]>
---
 drivers/nvdimm/e820.c | 1 +
 1 file changed, 1 insertion(+)

Patch

diff --git a/drivers/nvdimm/e820.c b/drivers/nvdimm/e820.c
index 4cd18be9d0e9..3af63b7b6d23 100644
--- a/drivers/nvdimm/e820.c
+++ b/drivers/nvdimm/e820.c
@@ -28,6 +28,7 @@  static int e820_register_one(struct resource *res, void *data)
 	ndr_desc.numa_node = numa_map_to_online_node(nid);
 	ndr_desc.target_node = nid;
 	set_bit(ND_REGION_PAGEMAP, &ndr_desc.flags);
+	set_bit(ND_REGION_PERSIST_CACHE, &ndr_desc.flags);
 	if (!nvdimm_pmem_region_create(nvdimm_bus, &ndr_desc))
 		return -ENXIO;
 	return 0;