powerpc/64/kdump: Limit kdump base to 512MB

Since commit e641eb03 ("powerpc: Fix up the kdump base cap to
128M") memory for kdump kernel has been reserved at an offset of 128MB.
This held up well for a long time before running into boot failure on
LPARs having a lot of cores. Commit 7c5ed82b ("powerpc: Set
crashkernel offset to mid of RMA region") fixed this boot failure by
moving the offset to mid of RMA region. This change meant the offset is
either 256MB or 512MB on LPARs as ppc64_rma_size was 512MB or 1024MB
owing to commit 103a8542 ("powerpc/book3s64/ radix: Fix boot
failure with large amount of guest memory").

But ppc64_rma_size can be larger as well with newer f/w. So, limit
crashkernel reservation offset to 512MB to avoid running into boot
failures during kdump kernel boot, due to RTAS or other allocation
restrictions.

Also, while here, use SZ_128M instead of opening coding it.
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220912065031.57416-1-hbathini@linux.ibm.com

arch/powerpc/kexec/core.c

@@ -136,7 +136,7 @@ void __init reserve_crashkernel(void)
 #ifdef CONFIG_PPC64
 		/*
 		 * On the LPAR platform place the crash kernel to mid of
-		 * RMA size (512MB or more) to ensure the crash kernel
+		 * RMA size (max. of 512MB) to ensure the crash kernel
 		 * gets enough space to place itself and some stack to be
 		 * in the first segment. At the same time normal kernel
 		 * also get enough space to allocate memory for essential
@@ -144,9 +144,9 @@ void __init reserve_crashkernel(void)
 		 * kernel starts at 128MB offset on other platforms.
 		 */
 		if (firmware_has_feature(FW_FEATURE_LPAR))
-			crashk_res.start = ppc64_rma_size / 2;
+			crashk_res.start = min_t(u64, ppc64_rma_size / 2, SZ_512M);
 		else
-			crashk_res.start = min(0x8000000ULL, (ppc64_rma_size / 2));
+			crashk_res.start = min_t(u64, ppc64_rma_size / 2, SZ_128M);
 #else
 		crashk_res.start = KDUMP_KERNELBASE;
 #endif