[PATCH v6 6/8] powerpc/pmem: Avoid the barrier in flush routines

[Dan Williams]

On Tue, Jun 30, 2020 at 2:21 AM Aneesh Kumar K.V
<aneesh.kumar at linux.ibm.com> wrote:
[..]
> >> The bio argument isn't for range based flushing, it is for flush
> >> operations that need to complete asynchronously.
> > How does the block layer determine that the pmem device needs
> > asynchronous fushing?
> >
>
>         set_bit(ND_REGION_ASYNC, &ndr_desc.flags);
>
> and dax_synchronous(dev)

Yes, but I think it is overkill to have an indirect function call just
for a single instruction.

How about something like this instead, to share a common pmem_wmb()
across x86 and powerpc.

diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
index 20ff30c2ab93..b14009060c83 100644
--- a/drivers/nvdimm/region_devs.c
+++ b/drivers/nvdimm/region_devs.c
@@ -1180,6 +1180,13 @@ int nvdimm_flush(struct nd_region *nd_region,
struct bio *bio)
 {
        int rc = 0;

+       /*
+        * pmem_wmb() is needed to 'sfence' all previous writes such
+        * that they are architecturally visible for the platform buffer
+        * flush.
+        */
+       pmem_wmb();
+
        if (!nd_region->flush)
                rc = generic_nvdimm_flush(nd_region);
        else {
@@ -1206,17 +1213,14 @@ int generic_nvdimm_flush(struct nd_region *nd_region)
        idx = this_cpu_add_return(flush_idx, hash_32(current->pid + idx, 8));

        /*
-        * The first wmb() is needed to 'sfence' all previous writes
-        * such that they are architecturally visible for the platform
-        * buffer flush.  Note that we've already arranged for pmem
-        * writes to avoid the cache via memcpy_flushcache().  The final
-        * wmb() ensures ordering for the NVDIMM flush write.
+        * Note that we've already arranged for pmem writes to avoid the
+        * cache via memcpy_flushcache().  The final wmb() ensures
+        * ordering for the NVDIMM flush write.
         */
-       wmb();
        for (i = 0; i < nd_region->ndr_mappings; i++)
                if (ndrd_get_flush_wpq(ndrd, i, 0))
                        writeq(1, ndrd_get_flush_wpq(ndrd, i, idx));
-       wmb();
+       pmem_wmb();

        return 0;
 }