Commit ca8b5d97 authored by Nicolas Pitre's avatar Nicolas Pitre
Browse files

ARM: XIP kernel: store .data compressed in ROM

The .data segment stored in ROM is only copied to RAM once at boot time
and never referenced afterwards. This is arguably a suboptimal usage of
ROM resources.

This patch allows for compressing the .data segment before storing it
into ROM and decompressing it to RAM rather than simply copying it,
saving on precious ROM space.

Because global data is not available yet (obviously) we must allocate
decompressor workspace memory on the stack. The .bss area is used as a
stack area for that purpose before it is cleared. The required stack
frame is 9568 bytes for __inflate_kernel_data() alone, so make sure
the .bss is large enough to cope with that plus extra room for called
functions or fail the build.

Those numbers were picked arbitrarily based on the above 9568 byte
stack frame:

10240 (2.5 * PAGE_SIZE): used to override -Wframe-larger-than whose
default value is 1024.
12288 (3 * PAGE_SIZE): minimum .bss size to contain the stack.

Signed-off-by: default avatarNicolas Pitre <>
Reviewed-by: default avatarArd Biesheuvel <>
Tested-by: default avatarChris Brandt <>
parent 0d302c71
......@@ -2005,6 +2005,17 @@ config XIP_PHYS_ADDR
be linked for and stored to. This address is dependent on your
own flash usage.
bool "Store kernel .data section compressed in ROM"
depends on XIP_KERNEL
Before the kernel is actually executed, its .data section has to be
copied to RAM from ROM. This option allows for storing that data
in compressed form and decompressed to RAM rather than merely being
copied, saving some precious ROM space. A possible drawback is a
slightly longer boot delay.
config KEXEC
bool "Kexec system call (EXPERIMENTAL)"
depends on (!SMP || PM_SLEEP_SMP)
......@@ -31,8 +31,19 @@ targets := Image zImage xipImage bootpImage uImage
cmd_deflate_xip_data = $(CONFIG_SHELL) -c \
'$(srctree)/$(src)/ $< $@ || { rm -f $@; false; }'
quiet_cmd_mkxip = XIPZ $@
cmd_mkxip = $(cmd_objcopy) && $(cmd_deflate_xip_data)
quiet_cmd_mkxip = $(quiet_cmd_objcopy)
cmd_mkxip = $(cmd_objcopy)
$(obj)/xipImage: vmlinux FORCE
$(call if_changed,objcopy)
$(call if_changed,mkxip)
@$(kecho) ' Physical Address of xipImage: $(CONFIG_XIP_PHYS_ADDR)'
$(obj)/Image $(obj)/zImage: FORCE
# XIP kernel .data segment compressor
# Created by: Nicolas Pitre, August 2017
# Copyright: (C) 2017 Linaro Limited
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License version 2 as
# published by the Free Software Foundation.
# This script locates the start of the .data section in xipImage and
# substitutes it with a compressed version. The needed offsets are obtained
# from symbol addresses in vmlinux. It is expected that .data extends to
# the end of xipImage.
set -e
DD="dd status=none"
# Use "make V=1" to debug this script.
set -x
sym_val() {
# extract hex value for symbol in $1
local val=$($NM "$VMLINUX" | sed -n "/ $1$/{s/ .*$//p;q}")
[ "$val" ] || { echo "can't find $1 in $VMLINUX" 1>&2; exit 1; }
# convert from hex to decimal
echo $((0x$val))
__data_loc=$(sym_val __data_loc)
_edata_loc=$(sym_val _edata_loc)
base_offset=$(sym_val _xiprom)
# convert to file based offsets
data_start=$(($__data_loc - $base_offset))
data_end=$(($_edata_loc - $base_offset))
# Make sure data occupies the last part of the file.
file_end=$(stat -c "%s" "$XIPIMAGE")
if [ "$file_end" != "$data_end" ]; then
printf "end of xipImage doesn't match with _edata_loc (%#x vs %#x)\n" \
$(($file_end + $base_offset)) $_edata_loc 2>&1
exit 1;
# be ready to clean up
trap 'rm -f "$XIPIMAGE.tmp"' 0 1 2 3
# substitute the data section by a compressed version
$DD if="$XIPIMAGE" count=$data_start iflag=count_bytes of="$XIPIMAGE.tmp"
$DD if="$XIPIMAGE" skip=$data_start iflag=skip_bytes |
gzip -9 >> "$XIPIMAGE.tmp"
# replace kernel binary
mv -f "$XIPIMAGE.tmp" "$XIPIMAGE"
......@@ -87,6 +87,11 @@ head-y := head$(MMUEXT).o
obj-$(CONFIG_DEBUG_LL) += debug.o
obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
# This is executed very early using a temporary stack when no memory allocator
# nor global data is available. Everything has to be allocated on the stack.
CFLAGS_head-inflate-data.o := $(call cc-option,-Wframe-larger-than=10240)
obj-$(CONFIG_XIP_DEFLATED_DATA) += head-inflate-data.o
obj-$(CONFIG_ARM_VIRT_EXT) += hyp-stub.o
AFLAGS_hyp-stub.o :=-Wa,-march=armv7-a
ifeq ($(CONFIG_ARM_PSCI),y)
......@@ -87,7 +87,14 @@ __mmap_switched:
adr r4, __mmap_switched_data
mov fp, #0
ARM( ldr sp, [r4], #4 )
THUMB( ldr sp, [r4] )
THUMB( add r4, #4 )
bl __inflate_kernel_data @ decompress .data to RAM
teq r0, #0
bne __error
#elif defined(CONFIG_XIP_KERNEL)
ARM( ldmia r4!, {r0, r1, r2, sp} )
THUMB( ldmia r4!, {r0, r1, r2, r3} )
THUMB( mov sp, r3 )
......@@ -114,9 +121,11 @@ ENDPROC(__mmap_switched)
.type __mmap_switched_data, %object
.long _sdata @ r0
.long __data_loc @ r1
.long _edata_loc @ r2
.long __bss_stop @ sp (temporary stack in .bss)
* XIP kernel .data segment decompressor
* Created by: Nicolas Pitre, August 2017
* Copyright: (C) 2017 Linaro Limited
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
#include <linux/init.h>
#include <linux/zutil.h>
/* for struct inflate_state */
#include "../../../lib/zlib_inflate/inftrees.h"
#include "../../../lib/zlib_inflate/inflate.h"
#include "../../../lib/zlib_inflate/infutil.h"
extern char __data_loc[];
extern char _edata_loc[];
extern char _sdata[];
* This code is called very early during the boot process to decompress
* the .data segment stored compressed in ROM. Therefore none of the global
* variables are valid yet, hence no kernel services such as memory
* allocation is available. Everything must be allocated on the stack and
* we must avoid any global data access. We use a temporary stack located
* in the .bss area. The linker script makes sure the .bss is big enough
* to hold our stack frame plus some room for called functions.
* We mimic the code in lib/decompress_inflate.c to use the smallest work
* area possible. And because everything is statically allocated on the
* stack then there is no need to clean up before returning.
int __init __inflate_kernel_data(void)
struct z_stream_s stream, *strm = &stream;
struct inflate_state state;
char *in = __data_loc;
int rc;
/* Check and skip gzip header (assume no filename) */
if (in[0] != 0x1f || in[1] != 0x8b || in[2] != 0x08 || in[3] & ~3)
return -1;
in += 10;
strm->workspace = &state;
strm->next_in = in;
strm->avail_in = _edata_loc - __data_loc; /* upper bound */
strm->next_out = _sdata;
strm->avail_out = _edata_loc - __data_loc;
zlib_inflateInit2(strm, -MAX_WBITS);
WS(strm)->inflate_state.wsize = 0;
WS(strm)->inflate_state.window = NULL;
rc = zlib_inflate(strm, Z_FINISH);
if (rc == Z_OK || rc == Z_STREAM_END)
rc = strm->avail_out; /* should be 0 */
return rc;
......@@ -306,3 +306,11 @@ ASSERT((__arch_info_end - __arch_info_begin), "no machine record defined")
ASSERT(__hyp_idmap_text_end - (__hyp_idmap_text_start & PAGE_MASK) <= PAGE_SIZE,
"HYP init code too big or misaligned")
* The .bss is used as a stack area for __inflate_kernel_data() whose stack
* frame is 9568 bytes. Make sure it has extra room left.
ASSERT((_end - __bss_start) >= 12288, ".bss too small for CONFIG_XIP_DEFLATED_DATA")
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