[PATCH 4/7] LoongArch: kexec: avoid overwriting QEMU's machine FDT at 0x100000

[George Guo <dongtai.guo@linux.dev>]

From: George Guo <guodongtai@kylinos.cn>

QEMU places its machine FDT at physical address 0x100000 when booting
with '-kernel'.  KEXEC_CONTROL_CODE was defined at the same address, so
machine_kexec_prepare() overwrites the FDT with the relocation trampoline
before jumping to the new kernel.

The kexec'd kernel's fdt_setup() reads the FDT pointer from the EFI
config table (FDTPTR = 0x100000) and finds trampoline code instead of a
valid FDT, so earlycon auto-detection fails and the second kernel boots
silently with no console output.

Move KEXEC_CONTROL_CODE to 0x180000, which is still within the first 2MB
reserved by memblock_init() and does not conflict with the QEMU FDT.

Signed-off-by: George Guo <guodongtai@kylinos.cn>
---
 arch/loongarch/kernel/machine_kexec.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/loongarch/kernel/machine_kexec.c b/arch/loongarch/kernel/machine_kexec.c
index d7fafda1d541..44df7dbd3de5 100644
--- a/arch/loongarch/kernel/machine_kexec.c
+++ b/arch/loongarch/kernel/machine_kexec.c
@@ -21,8 +21,13 @@
 #include <asm/cacheflush.h>
 #include <asm/page.h>
 
-/* 0x100000 ~ 0x200000 is safe */
-#define KEXEC_CONTROL_CODE	TO_CACHE(0x100000UL)
+/*
+ * Both addresses are within the first 2MB which is always reserved by
+ * memblock_init().  Avoid 0x100000 because QEMU places its machine FDT
+ * there when using '-kernel'; overwriting it silences earlycon in the
+ * kexec'd kernel.
+ */
+#define KEXEC_CONTROL_CODE	TO_CACHE(0x180000UL)
 #define KEXEC_CMDLINE_ADDR	TO_CACHE(0x108000UL)
 
 static unsigned long reboot_code_buffer;
-- 
2.25.1