The simulator represents physical addresses using the address of its
allocated buffer. That buffer can lie above the default
ARCH_LOW_ADDRESS_LIMIT, preventing meaningful memblock_alloc_low()
tests.

Use the architecture override in asm/dma.h to place the limit halfway
through the memory registered by setup_memblock(). Resolve the limit
after allocating the buffer, leaving registered memory on both sides.

Assisted-by: LLM
Signed-off-by: Tianyi Chen <hi@tychen.cc>
---
 tools/testing/memblock/asm/dma.h      | 6 ++++++
 tools/testing/memblock/tests/common.c | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/tools/testing/memblock/asm/dma.h b/tools/testing/memblock/asm/dma.h
index 13ff8e5d22ef..6607408dcf25 100644
--- a/tools/testing/memblock/asm/dma.h
+++ b/tools/testing/memblock/asm/dma.h
@@ -2,4 +2,10 @@
 #ifndef _TOOLS_DMA_H
 #define _TOOLS_DMA_H
 
+#include <linux/types.h>
+
+phys_addr_t dummy_physical_memory_low_limit(void);
+
+#define ARCH_LOW_ADDRESS_LIMIT dummy_physical_memory_low_limit()
+
 #endif
diff --git a/tools/testing/memblock/tests/common.c b/tools/testing/memblock/tests/common.c
index 3250c8e5124b..ac032610a56e 100644
--- a/tools/testing/memblock/tests/common.c
+++ b/tools/testing/memblock/tests/common.c
@@ -118,6 +118,12 @@ phys_addr_t dummy_physical_memory_base(void)
 	return (phys_addr_t)memory_block.base;
 }
 
+phys_addr_t dummy_physical_memory_low_limit(void)
+{
+	/* The low limit is halfway through memory registered by setup_memblock(). */
+	return dummy_physical_memory_base() + MEM_SIZE / 2;
+}
+
 static void usage(const char *prog)
 {
 	BUILD_BUG_ON(ARRAY_SIZE(help_opts) != ARRAY_SIZE(long_opts) - 1);
-- 
2.55.0