[PATCH v2025.09.y 18/49] ARM: mmu: fix hang when reserved memory at start of RAM

[Ahmad Fatoum]

If the memory bank starts at address 0 and the start of the SDRAM is a
reserved region, remap_range_end_sans_text() is called with start = 0,
end = 0, which is interpreted by the function to mean that it spans
the whole of RAM.

Fix the issue by not calling remap_range_end_sans_text() for empty
ranges. This fixes an early hang observed on the Raspberry Pi 3.

In the unlikely case, there is no reserved memory region and the first
bank is indeed 4G of size, we also end up with start = 0, end = 0, but
this is already being handled correctly.

Of course, this fix introduces a corner case in return: A reserved memory
entry spanning the whole lower 4G becomes unsupported on 32-bit, but
that should be a non-issue  as such a system would be fairly useless and
64-bit is not affected...

A wholesale replacement of this function is under way, so we'll
hopefully get rid of this error prone code altogether in v2026.01.0.

Fixes: 3e2d06afabe1 ("range: fix corner cases when exclusive end is zero")
Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
Link: https://lore.barebox.org/20251107182750.3367036-1-a.fatoum@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>
(cherry picked from commit ee135d98586b68892add6d139929298b69da1878)
Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
---
 arch/arm/cpu/mmu-common.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm/cpu/mmu-common.c b/arch/arm/cpu/mmu-common.c
index 92e06b06077f..b3d9e9579686 100644
--- a/arch/arm/cpu/mmu-common.c
+++ b/arch/arm/cpu/mmu-common.c
@@ -149,7 +149,8 @@ static void mmu_remap_memory_banks(void)
 
 		/* Skip reserved regions */
 		for_each_reserved_region(bank, rsv) {
-			remap_range_end_sans_text(pos, rsv->start, MAP_CACHED);
+			if (pos != rsv->start)
+				remap_range_end_sans_text(pos, rsv->start, MAP_CACHED);
 			pos = rsv->end + 1;
 		}
 
-- 
2.47.3