[PATCH v2 1/3] mm/memblock: implement memblock_advise_size_order and probe functions

[Gregory Price <gourry@gourry.net>]

Hotplug memory sources may have opinions on what the memblock size
should be - usually for alignment purposes.  For example, CXL memory
extents can be as small as 256MB with a matching physical alignment.

Implement memblock_advise_size_order for use during early init, prior
to allocator and smp init, for software to advise the system as to what
the preferred block size should be.

The probe function is meant for arch_init code to fetch this value
once during memblock size calculation. Use of the advisement value
is arch-specific, and no guarantee is made that it will be used.

Calls to either function after probe results in -EBUSY to signal that
advisement is ignored or that memblock_get_size_bytes should be used.

Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Gregory Price <gourry@gourry.net>
---
 include/linux/memblock.h |  2 ++
 mm/memblock.c            | 49 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index fc4d75c6cec3..efb1f7cfbd58 100644
--- a/include/linux/memblock.h
+++ b/include/linux/memblock.h
@@ -111,6 +111,8 @@ static inline void memblock_discard(void) {}
 #endif
 
 void memblock_allow_resize(void);
+int memblock_advise_size_order(int order);
+int memblock_probe_size_order(void);
 int memblock_add_node(phys_addr_t base, phys_addr_t size, int nid,
 		      enum memblock_flags flags);
 int memblock_add(phys_addr_t base, phys_addr_t size);
diff --git a/mm/memblock.c b/mm/memblock.c
index 3b9dc2d89b8a..e0bdba011564 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2009,6 +2009,55 @@ void __init memblock_allow_resize(void)
 	memblock_can_resize = 1;
 }
 
+/*
+ * @order: bit-order describing the preferred minimum block size
+ *
+ * Intended for use by early-boot software prior to smp and allocator init to
+ * advise the architecture what the minimum block size should be. Should only
+ * be called during arch init before allocator and smp init.
+ *
+ * This value can only decrease after it has been initially set, the intention
+ * is to identify the smallest supported alignment across all opinions.
+ *
+ * Use of this advisement value is arch-specific.
+ *
+ * Returns: 0 on success, -EINVAL if order is <=0, and -EBUSY if already probed
+ */
+static int memblock_sz_order;
+#define MEMBLOCK_SZO_PROBED (-1)
+int memblock_advise_size_order(int order)
+{
+	if (order <= 0)
+		return -EINVAL;
+
+	if (memblock_sz_order == MEMBLOCK_SZO_PROBED)
+		return -EBUSY;
+
+	if (memblock_sz_order)
+		memblock_sz_order = min(order, memblock_sz_order);
+	else
+		memblock_sz_order = order;
+
+	return 0;
+}
+
+/*
+ * memblock_probe_size_order is intended for arch init code to probe one time,
+ * for a suggested memory block size.  After the first call, the result will
+ * always be -EBUSY. A late user should call memory_block_size_bytes instead to
+ * determine the actual block size in use.
+ *
+ * Should only be called during arch init prior to allocator and smp init.
+ *
+ * Returns: block size order, 0 if never set, or -EBUSY if previously probed.
+ */
+int memblock_probe_size_order(void)
+{
+	int rv = xchg(&memblock_sz_order, -1);
+
+	return (rv == -1) ? -EBUSY : rv;
+}
+
 static int __init early_memblock(char *p)
 {
 	if (p && strstr(p, "debug"))
-- 
2.43.0