MIPS: relocation: add relocation support

this patch i a port of following patch from u-boot with some additional
integration changes and fixes of original code:
| Subject: [PATCH] MIPS: Stop building position independent code
|
| U-Boot has up until now built with -fpic for the MIPS architecture,
| producing position independent code which uses indirection through a
| global offset table, making relocation fairly straightforward as it
| simply involves patching up GOT entries.
|
| Using -fpic does however have some downsides. The biggest of these is
| that generated code is bloated in various ways. For example, function
| calls are indirected through the GOT & the t9 register:
|
|   8f998064   lw     t9,-32668(gp)
|   0320f809   jalr   t9
|
| Without -fpic the call is simply:
|
|   0f803f01   jal    be00fc04 <puts>
|
| This is more compact & faster (due to the lack of the load & the
| dependency the jump has on its result). It is also easier to read &
| debug because the disassembly shows what function is being called,
| rather than just an offset from gp which would then have to be looked up
| in the ELF to discover the target function.
|
| Another disadvantage of -fpic is that each function begins with a
| sequence to calculate the value of the gp register, for example:
|
|   3c1c0004   lui    gp,0x4
|   279c3384   addiu  gp,gp,13188
|   0399e021   addu   gp,gp,t9
|
| Without using -fpic this sequence no longer appears at the start of each
| function, reducing code size considerably.
|
| This patch switches U-Boot from building with -fpic to building with
| -fno-pic, in order to gain the benefits described above. The cost of
| this is an extra step during the build process to extract relocation
| data from the ELF & write it into a new .rel section in a compact
| format, plus the added complexity of dealing with multiple types of
| relocation rather than the single type that applied to the GOT. The
| benefit is smaller, cleaner, more debuggable code. The relocate_code()
| function is reimplemented in C to handle the new relocation scheme,
| which also makes it easier to read & debug.
|
| Taking maltael_defconfig as an example the size of u-boot.bin built
| using the Codescape MIPS 2016.05-06 toolchain (gcc 4.9.2, binutils
| 2.24.90) shrinks from 254KiB to 224KiB.
|
| Signed-off-by: Paul Burton <paul.burton@imgtec.com>

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 18 insertions, 0 deletions

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index a4070cfe32..080f955c38 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -32,6 +32,24 @@ config PHYS_ADDR_T_64BIT
 
 menu "Machine selection"
 
+config MIPS_RELOCATION_TABLE_SIZE
+	hex "Relocation table size"
+	range 0x100 0x10000
+	default "0x8000"
+	---help---
+	  A table of relocation data will be appended to the Barebox binary
+	  and parsed in relocate_code() to fix up all offsets in the relocated
+	  Barebox.
+
+	  This option allows the amount of space reserved for the table to be
+	  adjusted in a range from 256 up to 64k. The default is 32k and should
+	  be ok in most cases. Reduce this value to shrink the size of U-Boot
+	  binary.
+
+	  The build will fail and a valid size suggested if this is too small.
+
+	  If unsure, leave at the default value.
+
 config BUILTIN_DTB
 	bool "link a DTB into the barebox image"
 	depends on OFTREE