common: memsize: add RAM size probe based on alias detection

Add probe_ram_size_by_alias() to detect RAM size by checking whether a
write to one address aliases to another address.

Compared to get_ram_size(), this function allows the caller to:
- limit probing to a small set of required accesses
- avoid touching reserved or already used memory regions
- handle non-linear alias patterns

On the iMX95 SoC, when used with LPDDR5, accesses beyond the end of an 8GB DDR
configuration do not alias to the expected linear wrap-around addresses.
Instead, the aliased addresses appear to follow a pattern related to the
DDRC bank and bank-group addresses mapping. Experimentally, the observed
pattern is:

Write        Read
y00000000 -> x0001c000
y00004000 -> x00018000
y00008000 -> x00014000
y0000c000 -> x00010000
y00010000 -> x0000c000
y00014000 -> x00008000
y00018000 -> x00004000
y0001c000 -> x00000000

This helper makes it possible to probe RAM size by explicitly specifying
the probed address and the expected alias address for each size check.

Signed-off-by: Emanuele Ghidoli <[email protected]>
Reviewed-by: Simon Glass <[email protected]>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/init.h b/include/init.h
index 1e375da4893..c31ebd83b85 100644
--- a/include/init.h
+++ b/include/init.h
@@ -14,6 +14,12 @@
 
 #include <linux/types.h>
 
+struct ram_alias_check {
+	void *probe_addr;
+	void *alias_addr;
+	long size;
+};
+
 /*
  * In case of the EFI app the UEFI firmware provides the low-level
  * initialisation.
@@ -88,6 +94,7 @@ int dram_init(void);
 int dram_init_banksize(void);
 
 long get_ram_size(long *base, long size);
+long probe_ram_size_by_alias(const struct ram_alias_check *checks);
 phys_size_t get_effective_memsize(void);
 
 int testdram(void);