uboot向kernel的传参机制——bootm与tags
最近阅读代码学习了uboot boot kernel的过程以及uboot如何传参给kernel，记录下来，与大家共享：
U-boot版本：2014.4
Kernel版本：3.4.55
一 uboot 如何启动 kernel
1 do_bootm
uboot下使用bootm命令启动内核镜像文件uImage，uImage是在zImage头添加了64字节的镜像信息供uboot解析使用，具体这64字节头的内容，我们在分析bootm命令的时候就会一一说到，那直接来看bootm命令。
在common/cmd_bootm.c中
int do_bootm(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
{
#ifdef CONFIG_NEEDS_MANUAL_RELOC
static int relocated = 0;
if (!relocated) {
int i;
/* relocate boot function table */
for (i = 0; i < ARRAY_SIZE(boot_os); i++)
if (boot_os[i] != NULL)
boot_os[i] += gd->reloc_off;
/* relocate names of sub-command table */
for (i = 0; i < ARRAY_SIZE(cmd_bootm_sub); i++)
cmd_bootm_sub[i].name += gd->reloc_off;
relocated = 1;
}
#endif
/* determine if we have a sub command */
argc--; argv++;
if (argc > 0) {
char *endp;
simple_strtoul(argv[0], &endp, 16);
/* endp pointing to NULL means that argv[0] was just a
* valid number, pass it along to the normal bootm processing
*
* If endp is ':' or '#' assume a FIT identifier so pass
* along for normal processing.
*
* Right now we assume the first arg should never be '-'
*/
if ((*endp != 0) && (*endp != ':') && (*endp != '#'))
return do_bootm_subcommand(cmdtp, flag, argc, argv);
}
return do_bootm_states(cmdtp, flag, argc, argv, BOOTM_STATE_START |
BOOTM_STATE_FINDOS | BOOTM_STATE_FINDOTHER |
BOOTM_STATE_LOADOS |
#if defined(CONFIG_PPC) || defined(CONFIG_MIPS)
BOOTM_STATE_OS_CMDLINE |
#endif
BOOTM_STATE_OS_PREP | BOOTM_STATE_OS_FAKE_GO |
BOOTM_STATE_OS_GO, &images, 1);
}
但是boot_os函数在uboot relocate时已经都拷贝了，所以感觉没必要在进行relocate。这个宏因此没有定义，直接走下面。
新版uboot对于boot kernel实现了一个类似状态机的机制，将整个过程分成很多个阶段，uboot将每个阶段称为subcommand，
核心函数是do_bootm_states，需要执行哪个阶段，就在do_bootm_states最后一个参数添加那个宏定义，如： BOOTM_STATE_START
do_bootm_subcommand是按照bootm参数来指定运行某一个阶段，也就是某一个subcommand
对于正常的uImage，bootm加tftp的load地址就可以。
2 do_bootm_states
这样会走到最后函数do_bootm_states，那就来看看核心函数do_bootm_states
static int do_bootm_states(cmd_tbl_t *cmdtp, int flag, int argc,
char * const argv[], int states, bootm_headers_t *images,
int boot_progress)
{
boot_os_fn *boot_fn;
ulong iflag = 0;
int ret = 0, need_boot_fn;
images->state |= states;
/*
* Work through the states and see how far we get. We stop on
* any error.
*/
if (states & BOOTM_STATE_START)
ret = bootm_start(cmdtp, flag, argc, argv);
首先将states存储在images的state中，因为states中有BOOTM_STATE_START，调用bootm_start.
3 第一阶段：bootm_start
static int bootm_start(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
{
memset((void *)&images, 0, sizeof(images));
images.verify = getenv_yesno("verify");
boot_start_lmb(&images);
bootstage_mark_name(BOOTSTAGE_ID_BOOTM_START, "bootm_start");
images.state = BOOTM_STATE_START;
return 0;
}
boot_start_lmb暂时还没弄明白，以后再搞清楚。
最后修改images.state为bootm start。
bootm_start主要工作是清空images，标志当前状态为bootm start。
4 第二阶段：bootm_find_os
由bootm_start返回后，do_bootm传了BOOTM_STATE_FINDOS，所以进入函数bootm_find_os
static int bootm_find_os(cmd_tbl_t *cmdtp, int flag, int argc,
char * const argv[])
{
const void *os_hdr;
/* get kernel image header, start address and length */
os_hdr = boot_get_kernel(cmdtp, flag, argc, argv,
&images, &images.os.image_start, &images.os.image_len);
if (images.os.image_len == 0) {
puts("ERROR: can't get kernel image!\n");
return 1;
}调用boot_get_kernel，函数较长，首先是获取image的load地址，如果bootm有参数，就是img_addr,之后如下：
bootstage_mark(BOOTSTAGE_ID_CHECK_MAGIC);
/* copy from dataflash if needed */
img_addr = genimg_get_image(img_addr);
/* check image type, for FIT images get FIT kernel node */
*os_data = *os_len = 0;
buf = map_sysmem(img_addr, 0);
这里说明我们的image可以在flash中用bootm直接起
map_sysmem为空函数，buf即为img_addr。
switch (genimg_get_format(buf)) {
case IMAGE_FORMAT_LEGACY:
printf("## Booting kernel from Legacy Image at %08lx ...\n",
img_addr);
hdr = image_get_kernel(img_addr, images->verify);
if (!hdr)
return NULL;
bootstage_mark(BOOTSTAGE_ID_CHECK_IMAGETYPE);
/* get os_data and os_len */
switch (image_get_type(hdr)) {
case IH_TYPE_KERNEL:
case IH_TYPE_KERNEL_NOLOAD:
*os_data = image_get_data(hdr);
*os_len = image_get_data_size(hdr);
break;
case IH_TYPE_MULTI:
image_multi_getimg(hdr, 0, os_data, os_len);
break;
case IH_TYPE_STANDALONE:
*os_data = image_get_data(hdr);
*os_len = image_get_data_size(hdr);
break;
default:
printf("Wrong Image Type for %s command\n",
cmdtp->name);
bootstage_error(BOOTSTAGE_ID_CHECK_IMAGETYPE);
return NULL;
}
/*
* copy image header to allow for image overwrites during
* kernel decompression.
*/
memmove(&images->legacy_hdr_os_copy, hdr,
sizeof(image_header_t));
/* save pointer to image header */
images->legacy_hdr_os = hdr;
images->legacy_hdr_valid = 1;
bootstage_mark(BOOTSTAGE_ID_DECOMP_IMAGE);
break;
首先来说明一下image header的格式，在代码中由image_header_t代表，如下：
typedef struct image_header {
__be32 ih_magic; /* Image Header Magic Number */
__be32 ih_hcrc; /* Image Header CRC Checksum */
__be32 ih_time; /* Image Creation Timestamp */
__be32 ih_size; /* Image Data Size */
__be32 ih_load; /* Data Load Address */
__be32 ih_ep; /* Entry Point Address */
__be32 ih_dcrc; /* Image Data CRC Checksum */
uint8_t ih_os; /* Operating System */
uint8_t ih_arch; /* CPU architecture */
uint8_t ih_type; /* Image Type */
uint8_t ih_comp; /* Compression Type */
uint8_t ih_name[IH_NMLEN]; /* Image Name */
} image_header_t;image_get_kernel则会来计算header的crc是否正确，然后获取image的type，根据type来获取os的len和data起始地址。
最后将hdr的数据拷贝到images的legacy_hdr_os_copy，防止kernel image在解压是覆盖掉hdr数据，保存hdr指针到legacy_hdr_os中，置位legacy_hdr_valid。
从boot_get_kernel中返回到bootm_find_os，继续往下：
switch (genimg_get_format(os_hdr)) {
case IMAGE_FORMAT_LEGACY:
images.os.type = image_get_type(os_hdr);
images.os.comp = image_get_comp(os_hdr);
images.os.os = image_get_os(os_hdr);
images.os.end = image_get_image_end(os_hdr);
images.os.load = image_get_load(os_hdr);
/* find kernel entry point */
if (images.legacy_hdr_valid) {
images.ep = image_get_ep(&images.legacy_hdr_os_copy);
} else {
puts("Could not find kernel entry point!\n");
return 1;
}
if (images.os.type == IH_TYPE_KERNEL_NOLOAD) {
images.os.load = images.os.image_start;
images.ep += images.os.load;
}
images.os.start = (ulong)os_hdr;
到这里bootm_find_os就结束了，主要工作是根据image的hdr来做crc，获取一些基本的os信息到images结构体中。
回到do_bootm_states中接下来调用bootm_find_other，
5 第三阶段：bootm_find_other
该函数大体看一下，对于legacy类型的image，获取查询是否有ramdisk，此处我们没有用单独的ramdisk，ramdisk是直接编译到kernel image中的。
回到do_bootm_states中接下来会调用bootm_load_os。
6 第四阶段：bootm_load_os
static int bootm_load_os(bootm_headers_t *images, unsigned long *load_end,
int boot_progress)
{
image_info_t os = images->os;
uint8_t comp = os.comp;
ulong load = os.load;
ulong blob_start = os.start;
ulong blob_end = os.end;
ulong image_start = os.image_start;
ulong image_len = os.image_len;
__maybe_unused uint unc_len = CONFIG_SYS_BOOTM_LEN;
int no_overlap = 0;
void *load_buf, *image_buf;
#if defined(CONFIG_LZMA) || defined(CONFIG_LZO)
int ret;
#endif /* defined(CONFIG_LZMA) || defined(CONFIG_LZO) */
const char *type_name = genimg_get_type_name(os.type);
load_buf = map_sysmem(load, unc_len);
image_buf = map_sysmem(image_start, image_len);
switch (comp) {
case IH_COMP_NONE:
if (load == blob_start || load == image_start) {
printf(" XIP %s ... ", type_name);
no_overlap = 1;
} else {
printf(" Loading %s ... ", type_name);
memmove_wd(load_buf, image_buf, image_len, CHUNKSZ);
}
*load_end = load + image_len;
break;#ifdef CONFIG_GZIP
case IH_COMP_GZIP:
printf(" Uncompressing %s ... ", type_name);
if (gunzip(load_buf, unc_len, image_buf, &image_len) != 0) {
puts("GUNZIP: uncompress, out-of-mem or overwrite "
"error - must RESET board to recover\n");
if (boot_progress)
bootstage_error(BOOTSTAGE_ID_DECOMP_IMAGE);
return BOOTM_ERR_RESET;
}
*load_end = load + image_len;
break;
#endif /* CONFIG_GZIP */
之后则是根据hdr的comp类型来解压拷贝image到load addr上。
这里就需要注意，kernel选项的压缩格式必须在uboot下打开相应的解压缩支持，或者就不进行压缩
这里还有一点，load addr与image add是否可以重叠，看代码感觉是可以重叠的，还需要实际测试一下。
回到do_bootm_states，接下来根据os从boot_os数组中获取到了相应的os boot func，这里是linux，则是do_bootm_linux。后面代码如下：
/* Call various other states that are not generally used */
if (!ret && (states & BOOTM_STATE_OS_CMDLINE))
ret = boot_fn(BOOTM_STATE_OS_CMDLINE, argc, argv, images);
if (!ret && (states & BOOTM_STATE_OS_BD_T))
ret = boot_fn(BOOTM_STATE_OS_BD_T, argc, argv, images);
if (!ret && (states & BOOTM_STATE_OS_PREP))
ret = boot_fn(BOOTM_STATE_OS_PREP, argc, argv, images);
。。。。
/* Check for unsupported subcommand. */
if (ret) {
puts("subcommand not supported\n");
return ret;
}
/* Now run the OS! We hope this doesn't return */
if (!ret && (states & BOOTM_STATE_OS_GO))
ret = boot_selected_os(argc, argv, BOOTM_STATE_OS_GO,
images, boot_fn);
int do_bootm_linux(int flag, int argc, char *argv[], bootm_headers_t *images)
{
/* No need for those on ARM */
if (flag & BOOTM_STATE_OS_BD_T || flag & BOOTM_STATE_OS_CMDLINE)
return -1;
if (flag & BOOTM_STATE_OS_PREP) {
boot_prep_linux(images);
return 0;
}
if (flag & (BOOTM_STATE_OS_GO | BOOTM_STATE_OS_FAKE_GO)) {
boot_jump_linux(images, flag);
return 0;
}
boot_prep_linux(images);
boot_jump_linux(images, flag);
return 0;
}
7 第五阶段：boot_prep_linux
该函数作用是为启动后的kernel准备参数，这个函数我们在第三部分uboot如何传参给kernel再仔细分析一下
boot_prep_linux完成返回到do_bootm_states后接下来就是最后一步了。执行boot_selected_os调用do_bootm_linux，flag为BOOTM_STATE_OS_GO,则调用boot_jump_linux
8 第六阶段：boot_jump_linux
unsigned long machid = gd->bd->bi_arch_number;
char *s;
void (*kernel_entry)(int zero, int arch, uint params);
unsigned long r2;
int fake = (flag & BOOTM_STATE_OS_FAKE_GO);
kernel_entry = (void (*)(int, int, uint))images->ep;
s = getenv("machid");
if (s) {
strict_strtoul(s, 16, &machid);
printf("Using machid 0x%lx from environment\n", machid);
}
debug("## Transferring control to Linux (at address %08lx)" \
"...\n", (ulong) kernel_entry);
bootstage_mark(BOOTSTAGE_ID_RUN_OS);
announce_and_cleanup(fake);
if (IMAGE_ENABLE_OF_LIBFDT && images->ft_len)
r2 = (unsigned long)images->ft_addr;
else
r2 = gd->bd->bi_boot_params;
if (!fake)
kernel_entry(0, machid, r2);
获取gd->bd->bi_arch_number为machid，如果有env则用env的machid，kernel_entry为之前由hdr获取的ep，也就是内核的入口地址。
fake为0，直接调用kernel_entry，参数1为0，参数2为machid，参数3为bi_boot_params。
这之后就进入了kernel的执行流程启动，就不会再回到uboot
这整个boot过程中bootm_images_t一直作为对image信息的全局存储结构。
三 uboot如何传参给kernel
uboot下的传参机制就直接来分析boot_prep_linux函数就可以了，如下：
static void boot_prep_linux(bootm_headers_t *images)
{
char *commandline = getenv("bootargs");
if (IMAGE_ENABLE_OF_LIBFDT && images->ft_len) {
#ifdef CONFIG_OF_LIBFDT
debug("using: FDT\n");
if (image_setup_linux(images)) {
printf("FDT creation failed! hanging...");
hang();
}
#endif
} else if (BOOTM_ENABLE_TAGS) {
debug("using: ATAGS\n");
setup_start_tag(gd->bd);
if (BOOTM_ENABLE_SERIAL_TAG)
setup_serial_tag(¶ms);
if (BOOTM_ENABLE_CMDLINE_TAG)
setup_commandline_tag(gd->bd, commandline);
if (BOOTM_ENABLE_REVISION_TAG)
setup_revision_tag(¶ms);
if (BOOTM_ENABLE_MEMORY_TAGS)
setup_memory_tags(gd->bd);
if (BOOTM_ENABLE_INITRD_TAG) {
if (images->rd_start && images->rd_end) {
setup_initrd_tag(gd->bd, images->rd_start,
images->rd_end);
}
}
setup_board_tags(¶ms);
setup_end_tag(gd->bd);
} else {
printf("FDT and ATAGS support not compiled in - hanging\n");
hang();
}
do_nonsec_virt_switch();
}
在配置文件中定义了CONFIG_CMDLINE_TAG以及CONFIG_SETUP_MEMORY_TAGS，根据arch/arm/include/asm/bootm.h，则会定义BOOTM_ENABLE_TAGS，首先调用setup_start_tag，如下：
static void setup_start_tag (bd_t *bd)
{
params = (struct tag *)bd->bi_boot_params;
params->hdr.tag = ATAG_CORE;
params->hdr.size = tag_size (tag_core);
params->u.core.flags = 0;
params->u.core.pagesize = 0;
params->u.core.rootdev = 0;
params = tag_next (params);
}
这里params为struct tag的结构，如下：
struct tag {
struct tag_header hdr;
union {
struct tag_core core;
struct tag_mem32 mem;
struct tag_videotext videotext;
struct tag_ramdisk ramdisk;
struct tag_initrd initrd;
struct tag_serialnr serialnr;
struct tag_revision revision;
struct tag_videolfb videolfb;
struct tag_cmdline cmdline;
/*
* Acorn specific
*/
struct tag_acorn acorn;
/*
* DC21285 specific
*/
struct tag_memclk memclk;
} u;
};
setup_start_tag是初始化了第一个tag，是tag_core类型的tag。最后调用tag_next跳到第一个tag末尾，为下一个tag做准备。
回到boot_prep_linux，接下来调用setup_commandline_tag，如下：
static void setup_commandline_tag(bd_t *bd, char *commandline)
{
char *p;
if (!commandline)
return;
/* eat leading white space */
for (p = commandline; *p == ' '; p++);
/* skip non-existent command lines so the kernel will still
* use its default command line.
*/
if (*p == '\0')
return;
params->hdr.tag = ATAG_CMDLINE;
params->hdr.size =
(sizeof (struct tag_header) + strlen (p) + 1 + 4) >> 2;
strcpy (params->u.cmdline.cmdline, p);
params = tag_next (params);
}
回到boot_prep_linux，调用setup_memory_tag，如下：
static void setup_memory_tags(bd_t *bd)
{
int i;
for (i = 0; i < CONFIG_NR_DRAM_BANKS; i++) {
params->hdr.tag = ATAG_MEM;
params->hdr.size = tag_size (tag_mem32);
params->u.mem.start = bd->bi_dram[i].start;
params->u.mem.size = bd->bi_dram[i].size;
params = tag_next (params);
}
}
回到boot_prep_linux中，调用setup_board_tags，这个函数是__weak属性，我们可以在自己的板级文件中去实现来保存跟板子相关的参数，如果没有实现，则是空函数。
最后调用setup_end_tags，如下：
static void setup_end_tag(bd_t *bd)
{
params->hdr.tag = ATAG_NONE;
params->hdr.size = 0;
}
这样整个参数的准备就结束了，最后在调用boot_jump_linux时会将tags的首地址也就是bi_boot_params传给kernel，供kernel来解析这些tag，kernel如何解析看第四部分kenrel如何找到并解析参数
总结一下，uboot将参数以tag数组的形式布局在内存的某一个地址，每个tag代表一种类型的参数，首尾tag标志开始和结束，首地址传给kernel供其解析。
四 kernel如何找到并解析参数
uboot在调用boot_jump_linux时最后kernel_entry(0, machid, r2);
按照二进制规范eabi，machid存在寄存器r1，r2即tag的首地址存在寄存器r2.
查看kernel的入口函数，在arch/arm/kernel/head.S，中可以看到如下一段汇编：
/*
* r1 = machine no, r2 = atags or dtb,
* r8 = phys_offset, r9 = cpuid, r10 = procinfo
*/
bl __vet_atags
__vet_atags:
tst r2, #0x3 @ aligned?
bne 1f
ldr r5, [r2, #0]
#ifdef CONFIG_OF_FLATTREE
ldr r6, =OF_DT_MAGIC @ is it a DTB?
cmp r5, r6
beq 2f
#endif
cmp r5, #ATAG_CORE_SIZE @ is first tag ATAG_CORE?
cmpne r5, #ATAG_CORE_SIZE_EMPTY
bne 1f
ldr r5, [r2, #4]
ldr r6, =ATAG_CORE
cmp r5, r6
bne 1f
2: mov pc, lr @ atag/dtb pointer is ok
1: mov r2, #0
mov pc, lr
ENDPROC(__vet_atags)
之后对参数的真正分析处理是在start_kernel的setup_arch中，在arch/arm/kernel/setup.c中，如下：
void __init setup_arch(char **cmdline_p)
{
struct machine_desc *mdesc;
setup_processor();
mdesc = setup_machine_fdt(__atags_pointer);
if (!mdesc)
mdesc = setup_machine_tags(machine_arch_type);
machine_desc = mdesc;
machine_name = mdesc->name;
#ifdef CONFIG_ZONE_DMA
if (mdesc->dma_zone_size) {
extern unsigned long arm_dma_zone_size;
arm_dma_zone_size = mdesc->dma_zone_size;
}
#endif
if (mdesc->restart_mode)
reboot_setup(&mdesc->restart_mode);
init_mm.start_code = (unsigned long) _text;
init_mm.end_code = (unsigned long) _etext;
init_mm.end_data = (unsigned long) _edata;
init_mm.brk = (unsigned long) _end;
/* populate cmd_line too for later use, preserving boot_command_line */
strlcpy(cmd_line, boot_command_line, COMMAND_LINE_SIZE);
*cmdline_p = cmd_line;
parse_early_param();
static struct machine_desc * __init setup_machine_tags(unsigned int nr)
{
struct tag *tags = (struct tag *)&init_tags;
struct machine_desc *mdesc = NULL, *p;
char *from = default_command_line;
。。。。
if (__atags_pointer)
tags = phys_to_virt(__atags_pointer);
else if (mdesc->atag_offset)
tags = (void *)(PAGE_OFFSET + mdesc->atag_offset);
。。。。。
if (tags->hdr.tag == ATAG_CORE) {
if (meminfo.nr_banks != 0)
squash_mem_tags(tags);
save_atags(tags);
parse_tags(tags);
}
/* parse_early_param needs a boot_command_line */
strlcpy(boot_command_line, from, COMMAND_LINE_SIZE);
。。。
}static int __init parse_tag(const struct tag *tag)
{
extern struct tagtable __tagtable_begin, __tagtable_end;
struct tagtable *t;
for (t = &__tagtable_begin; t < &__tagtable_end; t++)
if (tag->hdr.tag == t->tag) {
t->parse(tag);
break;
}
return t < &__tagtable_end;
}
/*
* Parse all tags in the list, checking both the global and architecture
* specific tag tables.
*/
static void __init parse_tags(const struct tag *t)
{
for (; t->hdr.size; t = tag_next(t))
if (!parse_tag(t))
printk(KERN_WARNING
"Ignoring unrecognised tag 0x%08x\n",
t->hdr.tag);
}
这个tagtable的处理函数是在调用__tagtable来注册的，如下：
static int __init parse_tag_cmdline(const struct tag *tag)
{
#if defined(CONFIG_CMDLINE_EXTEND)
strlcat(default_command_line, " ", COMMAND_LINE_SIZE);
strlcat(default_command_line, tag->u.cmdline.cmdline,
COMMAND_LINE_SIZE);
#elif defined(CONFIG_CMDLINE_FORCE)
pr_warning("Ignoring tag cmdline (using the default kernel command line)\n");
#else
strlcpy(default_command_line, tag->u.cmdline.cmdline,
COMMAND_LINE_SIZE);
#endif
return 0;
}
__tagtable(ATAG_CMDLINE, parse_tag_cmdline);
这样遍历并处理完tags list之后回到setup_machine_tags，将from（即default_command_line）中的cmdline拷贝到boot_command_line,
最后返回到setup_arch中，
/* populate cmd_line too for later use, preserving boot_command_line */
strlcpy(cmd_line, boot_command_line, COMMAND_LINE_SIZE);
*cmdline_p = cmd_line;
parse_early_param();
到这里kernel就完全接收并分析完成了uboot传过来的args。
简单的讲，uboot利用函数指针及传参规范，它将
l R0: 0x0
l R1: 机器号
l R2: 参数地址
三个参数传递给内核。
其中，R2寄存器传递的是一个指针，这个指针指向一个TAG区域。
UBOOT和Linux内核之间正是通过这个扩展了的TAG区域来进行复杂参数的传递，如 command line，文件系统信息等等，用户也可以扩展这个TAG来进行更多参数的传递。TAG区域的首地址，正是R2的值。