[RFC 1/1] ARM: bootm: recalculate decompression space

[Stefan Müller-Klieser]

According to the kernel documentation it is recommended to place the
compressed image between 32MiB and 128MiB. We will conform to this if we
have at least 64MiB of RAM. If this is not the case, we fall back to the
old scheme but take the relocated image into account.
This is required because of the ARM default kernel config changes
regarding RODATA layout, which lead to an increased compression factor
of the kernel image.

Signed-off-by: Stefan Müller-Klieser <s.mueller-klieser at phytec.de>
---
 arch/arm/lib/bootm.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/arch/arm/lib/bootm.c b/arch/arm/lib/bootm.c
index 803aa94..d4fe9a4 100644
--- a/arch/arm/lib/bootm.c
+++ b/arch/arm/lib/bootm.c
@@ -73,18 +73,24 @@ static int get_kernel_addresses(size_t image_size,
 {
 	unsigned long mem_start, mem_size;
 	int ret;
-	size_t image_decomp_size;
-	unsigned long spacing;
+	unsigned long decomp_space, spacing;
 
 	ret = sdram_start_and_size(&mem_start, &mem_size);
 	if (ret)
 		return ret;
 
 	/*
-	 * We don't know the exact decompressed size so just use a conservative
-	 * default of 4 times the size of the compressed image.
+	 * The kernel documentation "Documentation/arm/Booting" advises
+	 * to place the compressed image outside of the lowest 32MiB to
+	 * avoid relocation. We should do this if we have at least 64MiB
+	 * of ram. If we have less space, we assume a maximum
+	 * compression factor of 4 plus 1. The latter factor is the
+	 * space for the relocated image.
 	 */
-	image_decomp_size = PAGE_ALIGN(image_size * 4);
+	if (mem_size >= SZ_64M)
+		decomp_space = SZ_32M;
+	else
+		decomp_space = PAGE_ALIGN(image_size * 5);
 
 	/*
 	 * By default put oftree/initrd close behind compressed kernel image to
@@ -97,21 +103,21 @@ static int get_kernel_addresses(size_t image_size,
 		 * Place the kernel at an address where it does not need to
 		 * relocate itself before decompression.
 		 */
-		*load_address = mem_start + image_decomp_size;
+		*load_address = mem_start + decomp_space;
 		if (verbose)
 			printf("no OS load address, defaulting to 0x%08lx\n",
 				*load_address);
-	} else if (*load_address <= mem_start + image_decomp_size) {
+	} else if (*load_address <= mem_start + decomp_space) {
 		/*
 		 * If the user/image specified an address where the kernel needs
 		 * to relocate itself before decompression we need to extend the
 		 * spacing to allow this relocation to happen without
 		 * overwriting anything placed behind the kernel.
 		 */
-		spacing += image_decomp_size;
+		spacing += decomp_space;
 	}
 
-	*mem_free = PAGE_ALIGN(*load_address + image_size + spacing);
+	*mem_free = PAGE_ALIGN(*load_address + decomp_space + spacing);
 
 	return 0;
 }
-- 
1.9.1