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![Paul Burton](/assets/img/avatar_default.png)
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> Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com> Cc: u-boot@lists.denx.de Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com> Tested-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
165 lines
4.7 KiB
C
165 lines
4.7 KiB
C
/*
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* MIPS Relocation
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*
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* Copyright (c) 2017 Imagination Technologies Ltd.
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*
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* SPDX-License-Identifier: GPL-2.0+
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*
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* Relocation data, found in the .rel section, is generated by the mips-relocs
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* tool & contains a record of all locations in the U-Boot binary that need to
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* be fixed up during relocation.
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*
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* The data is a sequence of unsigned integers, which are of somewhat arbitrary
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* size. This is achieved by encoding integers as a sequence of bytes, each of
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* which contains 7 bits of data with the most significant bit indicating
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* whether any further bytes need to be read. The least significant bits of the
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* integer are found in the first byte - ie. it somewhat resembles little
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* endian.
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*
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* Each pair of two integers represents a relocation that must be applied. The
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* first integer represents the type of relocation as a standard ELF relocation
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* type (ie. R_MIPS_*). The second integer represents the offset at which to
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* apply the relocation, relative to the previous relocation or for the first
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* relocation the start of the relocated .text section.
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*
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* The end of the relocation data is indicated when type R_MIPS_NONE (0) is
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* read, at which point no further integers should be read. That is, the
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* terminating R_MIPS_NONE reloc includes no offset.
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*/
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#include <common.h>
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#include <asm/relocs.h>
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#include <asm/sections.h>
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/**
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* read_uint() - Read an unsigned integer from the buffer
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* @buf: pointer to a pointer to the reloc buffer
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*
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* Read one whole unsigned integer from the relocation data pointed to by @buf,
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* advancing @buf past the bytes encoding the integer.
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*
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* Returns: the integer read from @buf
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*/
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static unsigned long read_uint(uint8_t **buf)
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{
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unsigned long val = 0;
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unsigned int shift = 0;
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uint8_t new;
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do {
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new = *(*buf)++;
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val |= (new & 0x7f) << shift;
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shift += 7;
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} while (new & 0x80);
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return val;
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}
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/**
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* apply_reloc() - Apply a single relocation
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* @type: the type of reloc (R_MIPS_*)
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* @addr: the address that the reloc should be applied to
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* @off: the relocation offset, ie. number of bytes we're moving U-Boot by
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*
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* Apply a single relocation of type @type at @addr. This function is
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* intentionally simple, and does the bare minimum needed to fixup the
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* relocated U-Boot - in particular, it does not check for overflows.
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*/
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static void apply_reloc(unsigned int type, void *addr, long off)
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{
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uint32_t u32;
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switch (type) {
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case R_MIPS_26:
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u32 = *(uint32_t *)addr;
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u32 = (u32 & GENMASK(31, 26)) |
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((u32 + (off >> 2)) & GENMASK(25, 0));
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*(uint32_t *)addr = u32;
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break;
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case R_MIPS_32:
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*(uint32_t *)addr += off;
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break;
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case R_MIPS_64:
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*(uint64_t *)addr += off;
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break;
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case R_MIPS_HI16:
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*(uint32_t *)addr += off >> 16;
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break;
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default:
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panic("Unhandled reloc type %u\n", type);
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}
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}
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/**
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* relocate_code() - Relocate U-Boot, generally from flash to DDR
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* @start_addr_sp: new stack pointer
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* @new_gd: pointer to relocated global data
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* @relocaddr: the address to relocate to
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*
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* Relocate U-Boot from its current location (generally in flash) to a new one
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* (generally in DDR). This function will copy the U-Boot binary & apply
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* relocations as necessary, then jump to board_init_r in the new build of
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* U-Boot. As such, this function does not return.
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*/
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void relocate_code(ulong start_addr_sp, gd_t *new_gd, ulong relocaddr)
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{
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unsigned long addr, length, bss_len;
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uint8_t *buf, *bss_start;
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unsigned int type;
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long off;
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/*
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* Ensure that we're relocating by an offset which is a multiple of
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* 64KiB, ie. doesn't change the least significant 16 bits of any
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* addresses. This allows us to discard R_MIPS_LO16 relocs, saving
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* space in the U-Boot binary & complexity in handling them.
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*/
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off = relocaddr - (unsigned long)__text_start;
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if (off & 0xffff)
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panic("Mis-aligned relocation\n");
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/* Copy U-Boot to RAM */
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length = __image_copy_end - __text_start;
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memcpy((void *)relocaddr, __text_start, length);
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/* Now apply relocations to the copy in RAM */
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buf = __rel_start;
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addr = relocaddr;
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while (true) {
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type = read_uint(&buf);
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if (type == R_MIPS_NONE)
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break;
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addr += read_uint(&buf) << 2;
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apply_reloc(type, (void *)addr, off);
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}
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/* Ensure the icache is coherent */
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flush_cache(relocaddr, length);
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/* Clear the .bss section */
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bss_start = (uint8_t *)((unsigned long)__bss_start + off);
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bss_len = (unsigned long)&__bss_end - (unsigned long)__bss_start;
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memset(bss_start, 0, bss_len);
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/* Jump to the relocated U-Boot */
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asm volatile(
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"move $29, %0\n"
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" move $4, %1\n"
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" move $5, %2\n"
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" move $31, $0\n"
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" jr %3"
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: /* no outputs */
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: "r"(start_addr_sp),
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"r"(new_gd),
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"r"(relocaddr),
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"r"((unsigned long)board_init_r + off));
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/* Since we jumped to the new U-Boot above, we won't get here */
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unreachable();
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}
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