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: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Chris Brandt <Chris.Brandt@renesas.com>
diff --git a/arch/arm/kernel/head-common.S b/arch/arm/kernel/head-common.S
index bf9c4e3..a25027b 100644
--- a/arch/arm/kernel/head-common.S
+++ b/arch/arm/kernel/head-common.S
@@ -87,7 +87,14 @@
adr r4, __mmap_switched_data
mov fp, #0
-#ifdef CONFIG_XIP_KERNEL
+#if defined(CONFIG_XIP_DEFLATED_DATA)
+ 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 @@
.type __mmap_switched_data, %object
__mmap_switched_data:
#ifdef CONFIG_XIP_KERNEL
+#ifndef CONFIG_XIP_DEFLATED_DATA
.long _sdata @ r0
.long __data_loc @ r1
.long _edata_loc @ r2
+#endif
.long __bss_stop @ sp (temporary stack in .bss)
#endif