关于imx6q uboot中的flash_header.S一些疑问

Document created by marvin_xie on Apr 2, 2018
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以下代码摘抄自:uboot 2009源码中board/Freescale/mx6q_sabreauto/flash_header.S

#include <config.h>
#include <asm/arch/mx6.h>

#ifdef CONFIG_FLASH_HEADER
#ifndef CONFIG_FLASH_HEADER_OFFSET
# error "Must define the offset of flash header"
#endif

#define CPU_2_BE_32(l) \
((((l) & 0x000000FF) << 24) | \
(((l) & 0x0000FF00) << 8) | \
(((l) & 0x00FF0000) >> 8) | \
(((l) & 0xFF000000) >> 24))

#define MXC_DCD_ITEM(i, addr, val) \
dcd_node_##i: \
.word CPU_2_BE_32(addr) ; \
.word CPU_2_BE_32(val) ; \

.section ".text.flasheader", "x"
b _start
.org CONFIG_FLASH_HEADER_OFFSET

ivt_header: .word 0x402000D1 /* Tag=0xD1, Len=0x0020, Ver=0x40 */
app_code_jump_v: .word _start
reserv1: .word 0x0
dcd_ptr: .word dcd_hdr
boot_data_ptr: .word boot_data
self_ptr: .word ivt_header
app_code_csf: .word 0x0
reserv2: .word 0x0

boot_data: .word TEXT_BASE
image_len: .word _end_of_copy - TEXT_BASE + CONFIG_FLASH_HEADER_OFFSET
plugin: .word 0x0

dcd_hdr: .word 0x40D802D2 /* Tag=0xD2, Len=90*8 + 4 + 4, Ver=0x40 */
write_dcd_cmd: .word 0x04D402CC /* Tag=0xCC, Len=90*8 + 4, Param=0x04 */

#include <config.h>
#include <asm/arch/mx6.h>

#ifdef CONFIG_FLASH_HEADER
#ifndef CONFIG_FLASH_HEADER_OFFSET
# error "Must define the offset of flash header"
#endif

#define CPU_2_BE_32(l) \
((((l) & 0x000000FF) << 24) | \
(((l) & 0x0000FF00) << 8) | \
(((l) & 0x00FF0000) >> 8) | \
(((l) & 0xFF000000) >> 24))

#define MXC_DCD_ITEM(i, addr, val) \
dcd_node_##i: \
.word CPU_2_BE_32(addr) ; \
.word CPU_2_BE_32(val) ; \

.section ".text.flasheader", "x"
b _start
.org CONFIG_FLASH_HEADER_OFFSET

ivt_header: .word 0x402000D1 /* Tag=0xD1, Len=0x0020, Ver=0x40 */
app_code_jump_v: .word _start
reserv1: .word 0x0
dcd_ptr: .word dcd_hdr
boot_data_ptr: .word boot_data
self_ptr: .word ivt_header
app_code_csf: .word 0x0
reserv2: .word 0x0

boot_data: .word TEXT_BASE
image_len: .word _end_of_copy - TEXT_BASE + CONFIG_FLASH_HEADER_OFFSET
plugin: .word 0x0

dcd_hdr: .word 0x40D802D2 /* Tag=0xD2, Len=90*8 + 4 + 4, Ver=0x40 */
write_dcd_cmd: .word 0x04D402CC /* Tag=0xCC, Len=90*8 + 4, Param=0x04 */

 

我的疑问是上面代码标红的部分的意义是什么?确切的说,IMX6Q既然规定了IVT在不同的boot devices中的偏移地址,比如我的应用场景是emmc,偏移地址是0x400(1K),那么我的uboot镜像完全可以按照:IVT+uboot本体的格式来构建,这样一来当使用mfg工具烧写uboot镜像时就可以用以下的命令来执行:

<CMD state="Updater" type="push" body="$ dd if=$FILE of=/dev/mmcblk0 bs=512 seek=2 ">write U-Boot to sd card</CMD>

而不是默认的命令(跳过uboot.bin前0x400的字节):

<CMD state="Updater" type="push" body="$ dd if=$FILE of=/dev/mmcblk0 bs=512 seek=2 skip=2">write U-Boot to sd card</CMD>

这样看,那么flash_header.S前面的0x400字节是不是多余的呢,还是有什么特别的用处,如果直接把这种uboot.bin烧写到emmc的0x400处(不跳过uboot.bin前0x400的字节,即b _start, .org CONFIG_FLASH_HEADER_OFFSET),那是不是就直接会调整到_start函数开始执行,而不会进行DCD相关的配置?

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