[PATCH v1] common: memsize: fix occasionally failing alias probing

[Emanuele Ghidoli]

From: Emanuele Ghidoli <emanuele.ghidoli at toradex.com>

probe_ram_size_by_alias() detects whether a probe address still aliases
a lower address by writing through one address and reading through the
other.

On i.MX95 this occasionally reported a false non-alias when the alias
read happened immediately after the write.

A memory barrier alone, mb(), was tested but did not make the failure go
away. This suggests that ordering the CPU accesses is not sufficient for
this probe, likely because the issue is in the path to the memory
controller rather than in the core itself.

Read the written address back before checking the alias address. This
appears to force the write to become observable at the probe address
before using the alias read to decide whether the tested address range
exists.

If the readback does not match the written pattern, restore the saved
value and continue with the next check. This keeps the probe robust for
addresses that do not reliably retain the test pattern.

Fixes: 0977448b45e2 ("common: memsize: add RAM size probe based on alias detection")
Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli at toradex.com>
---
 common/memsize.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/common/memsize.c b/common/memsize.c
index 3ecf9bac2aac..fd22f85a164e 100644
--- a/common/memsize.c
+++ b/common/memsize.c
@@ -145,6 +145,7 @@ long get_ram_size(long *base, long maxsize)
 long probe_ram_size_by_alias(const struct ram_alias_check *checks)
 {
 	long save[2];
+	long pat;
 	int dcache_en = 0;
 	long ret = 0;
 
@@ -161,12 +162,27 @@ long probe_ram_size_by_alias(const struct ram_alias_check *checks)
 		if (dcache_en)
 			dcache_flush_invalidate(s);
 
-		*d = ~save[0];
+		pat = ~save[0];
+		*d = pat;
 		sync();
 		if (dcache_en)
 			dcache_flush_invalidate(d);
 
-		if (*s != ~save[0])
+		/*
+		 * Make sure the test pattern is observable at the probe
+		 * address before checking whether it is also visible through
+		 * the alias address.
+		 */
+		if (*d != pat) {
+			*d = save[1];
+			sync();
+			if (dcache_en)
+				dcache_flush_invalidate(d);
+			checks++;
+			continue;
+		}
+
+		if (*s != pat)
 			ret = checks->size;
 
 		/* Restore content */
-- 
2.43.0