在上一部分提到过了，vivi作为bootloader，向内核传递启动参数是其本职工作之一。要把这个情景分析清楚，不仅仅需要分析vivi的参数机 制，而且要分析Linux kernel的接收机制。因为这是一个简单的通信过程，比起本科所学习的TCP/IP来简单的多，但是因为简单，所以在协议上并不规范，理解上反而不如 TCP/IP协议。下面就分为两个方面对此情景分析。
一、综述内核参数传递机制
现在内核参数传递机制有两种：一种是基于struct param_struct，这种已经比较老了。缺点是该结构每个成员的位置是固定的，受限比较大。另外一种就是新的struct tag way。说新是相对的，Linux kernel 2.4.x都希望采用这种tag的方式。关于这方面的资料，可以有如下参考（所给出的目录是基于linux-2.4.18的内核，以顶层Makefile 所在目录为当前目录。这里基于ARM架构的S3C2410，其他的SoC可以类比很容易得到）：
1、关于bootloader的理解--【Documentation/arm/booting】
此文档详细的讲述了bootloader的作用，具体内容如下：
[armlinux@lqm arm]$ cat Booting
Booting ARM Linux
=================
Author: Russell King
Date : 18 May 2002
The following documentation is relevant to 2.4.18-rmk6 and beyond.
In order to boot ARM Linux, you require a boot loader, which is a small
program that runs before the main kernel. The boot loader is expected
to initialise various devices, and eventually call the Linux kernel,
passing information to the kernel.
Essentially, the boot loader should provide (as a minimum) the
following:
1. Setup and initialise the RAM.
2. Initialise one serial port.
3. Detect the machine type.
4. Setup the kernel tagged list.
5. Call the kernel image.
1. Setup and initialise RAM
---------------------------
Existing boot loaders: MANDATORY
New boot loaders: MANDATORY
The boot loader is expected to find and initialise all RAM that the
kernel will use for volatile data storage in the system. It performs
this in a machine dependent manner. (It may use internal algorithms
to automatically locate and size all RAM, or it may use knowledge of
the RAM in the machine, or any other method the boot loader designer
sees fit.)
2. Initialise one serial port
-----------------------------
Existing boot loaders: OPTIONAL, RECOMMENDED
New boot loaders: OPTIONAL, RECOMMENDED
The boot loader should initialise and enable one serial port on the
target. This allows the kernel serial driver to automatically detect
which serial port it should use for the kernel console (generally
used for debugging purposes, or communication with the target.)
As an alternative, the boot loader can pass the relevant 'console='
option to the kernel via the tagged lists specifing the port, and
serial format options as described in
linux/Documentation/kernel-parameters.txt.
3. Detect the machine type
--------------------------
Existing boot loaders: OPTIONAL
New boot loaders: MANDATORY
The boot loader should detect the machine type its running on by some
method. Whether this is a hard coded value or some algorithm that
looks at the connected hardware is beyond the scope of this document.
The boot loader must ultimately be able to provide a MACH_TYPE_xxx
value to the kernel. (see linux/arch/arm/tools/mach-types).
4. Setup the kernel tagged list
-------------------------------
Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED
New boot loaders: MANDATORY
The boot loader must create and initialise the kernel tagged list.
A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.
The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag
has the size field set to '2' (0x00000002). The ATAG_NONE must set
the size field to zero.
Any number of tags can be placed in the list. It is undefined
whether a repeated tag appends to the information carried by the
previous tag, or whether it replaces the information in its
entirety; some tags behave as the former, others the latter.
The boot loader must pass at a minimum the size and location of
the system memory, and root filesystem location. Therefore, the
minimum tagged list should look:
+-----------+
base -> | ATAG_CORE | |
+-----------+ |
| ATAG_MEM | | increasing address
+-----------+ |
| ATAG_NONE | |
+-----------+ v
The tagged list should be stored in system RAM.
The tagged list must be placed in a region of memory where neither
the kernel decompressor nor initrd 'bootp' program will overwrite
it. The recommended placement is in the first 16KiB of RAM.
5. Calling the kernel image
---------------------------
Existing boot loaders: MANDATORY
New boot loaders: MANDATORY
There are two options for calling the kernel zImage. If the zImage
is stored in flash, and is linked correctly to be run from flash,
then it is legal for the boot loader to call the zImage in flash
directly.
The zImage may also be placed in system RAM (at any location) and
called there. Note that the kernel uses 16K of RAM below the image
to store page tables. The recommended placement is 32KiB into RAM.
In either case, the following conditions must be met:
- CPU register settings
r0 = 0,
r1 = machine type number discovered in (3) above.
r2 = physical address of tagged list in system RAM.
- CPU mode
All forms of interrupts must be disabled (IRQs and FIQs)
The CPU must be in SVC mode. (A special exception exists for Angel)
- Caches, MMUs
The MMU must be off.
Instruction cache may be on or off.
Data cache must be off.
- The boot loader is expected to call the kernel image by jumping
directly to the first instruction of the kernel image.
可以看出bootloader最少具备5项功能，上面比较清晰。可以看出，现在2.4的内核都是希望采用tagged list的方式来进行传递的，这里没有提到比较老的方式。这里要特别注意的是，r2 = physical address of tagged list in system RAM.，这里的“必须”是针对于tagged list而言的，如果采用param_struct，则并没有这个限制。这在后面将会详细分析，而这正是可能导致疑惑的地方。
2、参数传递数据结构的定义位置【include/asm/setup.h】，在这里就可以看到两种参数传递方式了。可以说，现在 bootloader和Linux kernel约定的参数传递机制就是这两种，必须严格按照这两种机制进行传输，否则的话，kernel可能因为无法识别bootloader传递过来的参 数而导致无法启动。关于这两种方式，在这里还有说明：
/*
* linux/include/asm/setup.h
*
* Copyright (C) 1997-1999 Russell King
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*
* Structure passed to kernel to tell it about the
* hardware it's running on. See linux/Documentation/arm/Setup
* for more info.
*
* NOTE:
* This file contains two ways to pass information from the boot
* loader to the kernel. The old struct param_struct is deprecated,
* but it will be kept in the kernel for 5 years from now
* (2001). This will allow boot loaders to convert to the new struct
* tag way.
*/
这说明，现在参数传递必须要采用tag方式，因为现在新的kernel已经不支持param_struct方式了。不幸的是，vivi还是采用的 param_struct方式。这里暂时以param_struct为主分析，考虑以后更新为tag方式。在这里你也可以参考 【Documentation/arm/setup】，里面有关于选项具体含义的详细说明。（在这里多说几句。Linux的Documentation是 一个很好的学习库，几乎所有的问题在这里都能有初步的解答。如果要想继续深入，那么就要读源代码了。学习上，先看README，然后翻阅 Documentation，无疑是一条捷径。而且，是否有完备的文档，也是判断这个软件是否优秀的重要标准。）
二、vivi设置Linux参数分析
上面对bootloader与Linux kernel之间参数传递的两种方式已经有了一个总体的理解。下面就来先看vivi部分如何设置Linux参数。
【init/main.c】boot_or_vivi()-->run_autoboot()-->exec_string("boot")
到此，也就是要执行boot命令。与命令相关部分都在【lib/command.c】中，找到boot_cmd，然后跟踪至【lib/boot_kernel.c】，boot的执行行为函数为command_boot()，继续分析：
【lib/boot_kernel.c】command_boot()-->
主要就是三步工作。
· 获取media_type。
media_type = get_param_value("media_type", &ret);
media_type是重要的，因为对于不同的存储介质，底层的驱动函数是不同的。通过media_type这个顶层抽象，实现了与底层驱动的联系。
[armlinux@lqm include]$ cat boot_kernel.h
#ifndef _VIVI_BOOT_KERNEL_H_
#define _VIVI_BOOT_KERNEL_H_
/*
* Media Type: A type of storage device that contains the linux kernel
*
* +----------------+-----------------------------------------+
* | Value(Integer) | Type |
* +----------------+-----------------------------------------+
* | 0 | UNKNOWN |
* | 1 | RAM |
* | 2 | NOR Flash Memory |
* | 3 | SMC (NAND Flash Memory) on the S3C2410 |
* +----------------+-----------------------------------------+
*/
enum {
MT_UNKNOWN = 0,
MT_RAM,
MT_NOR_FLASH,
MT_SMC_S3C2410
};
#endif /* _VIVI_BOOT_KERNEL_H_ */
上面就是vivi支持的media_type，现在此开发板是MT_SMC_S3C2410，也就是nand flash memory的选择部分。
·获取nand flash的kernel分区信息，为下载做好准备
kernel_part = get_mtd_partition("kernel");
if (kernel_part == NULL) {
printk("Can't find default 'kernel' partition\n");
return;
}
from = kernel_part->offset;
size = kernel_part->size;
这里获得了kernel所在nand flash的起始地址和大小。这里应该注意，虽然kernel_part->offset是偏移量，但是这个偏移是相对于0x00000000而 言，所以这时的offset就是对应的起始地址。当然，对nand flash来说，这里的地址并非是内存映射，需要做一系列的变化，具体是在nand_read_ll函数中，前面的基本实验已经做过了。
·启动内核
boot_kernel(from, size, media_type);
利用前面得到的media_type，from，size就可以来启动内核了，当然还有多步工作要去做。具体包括如下内容：
（1）获取内存基地址
boot_mem_base = get_param_value("boot_mem_base", &ret);
在vivi中，sdram是从0x30000000开始的，所以这里的boot_mem_base就是0x30000000.
（2）把kernel映象从nand flash复制到sdram的固定位置
to = boot_mem_base + LINUX_KERNEL_OFFSET;
printk("Copy linux kernel from 0x%08lx to 0x%08lx, size = 0x%08lx ... ",
from, to, size);
ret = copy_kernel_img(to, (char *)from, size, media_type);
这里LINUX_KERNEL_OFFSET是0x8000，关于为什么是0x8000，这是历史原因造成的，是Linux内核的一个约定，具体可以查看Linux内核的源代码中的arch/arm/kernel/head_armv.S，如下：
/*
* We place the page tables 16K below TEXTADDR. Therefore, we must make sure
* that TEXTADDR is correctly set. Currently, we expect the least significant
* "short" to be 0x8000, but we could probably relax this restriction to
* TEXTADDR > PAGE_OFFSET + 0x4000
*
* Note that swapper_pg_dir is the virtual address of the page tables, and
* pgtbl gives us a position-independent reference to these tables. We can
* do this because stext == TEXTADDR
*
* swapper_pg_dir, pgtbl and krnladr are all closely related.
*/
可以看出，TEXTADDR就是stext的地址，本开发板上为0x30008000，在0x30008000往下，会放置16K的页表，预计是 0x8000.不过此处可能会放松这个限制。另外，我们的一些参数也会放到内存起始区域。这在后面就可以看到。总之，这个地方的位置 boot_mem_base也就是kernel的第一条指令所在地，最后的程序跳转要跳到这个位置。
（3）验证magic number
if (*(ulong *)(to + 9*4) != LINUX_ZIMAGE_MAGIC) {
printk("Warning: this binary is not compressed linux kernel image\n");
printk("zImage magic = 0x%08lx\n", *(ulong *)(to + 9*4));
} else {
printk("zImage magic = 0x%08lx\n", *(ulong *)(to + 9*4));
}
这个地方是判断是否有zImage的存在，而zImage的判别的magic number为0x016f2818，这个也是和内核约定好的。你可以用ultra-edit32查看一下zImage，这是我的zImage的头的部分内容（注意，为小端存放格式）：
00000000h: 00 00 A0 E1 00 00 A0 E1 00 00 A0 E1 00 00 A0 E1
00000010h: 00 00 A0 E1 00 00 A0 E1 00 00 A0 E1 00 00 A0 E1
00000020h: 02 00 00 EA 18 28 6f 01 00 00 00 00 DB 86 09 00
至于为什么magic number在0x00000024这个位置，需要分析zImage是如何生成的，它的内容是什么，起始的几个字节是什么，这部分内容放到Linux kernel端进行深入分析。不过在这里应该提一句，此处的验证是考虑到Linux kernel相对比较大，而嵌入式系统的资源受限，为了节省资源，一般会将Linux kernel来压缩成zImage格式（识别方式就是在第9个字后有magic number0x016f2818）；但是应该明确，这步工作并非是必需的。因为如果内核比较小，为了加快启动速度，我可以不使用压缩的映象，直接采用非 压缩映象，那么vivi此处应该把无法找到maigc number的提示更改为printk("this binary is not compressed linux kernel image\n");。就Linux kernel来说，启动中支持压缩映象和非压缩映象两种启动方式，不管是那种启动方式，第一条指令的地址总是boot_mem_base，只不过放在这里 的指令并非一定是真正的kernel启动指令。这个在后面会详细分析Linux kernel启动方式。
（4）设置Linux参数
setup_linux_param(boot_mem_base + LINUX_PARAM_OFFSET);
现在看一下setup_linux_param的具体动作。
static void setup_linux_param(ulong param_base)
{
struct param_struct *params = (struct param_struct *)param_base;
char *linux_cmd;
//第一步：打印出param_base的基地址，这里就是0x30000100
//这里的这个位置实际上是约定的，预留了256字节
//然后初始化param_struct这个数据结构
printk("Setup linux parameters at 0x%08lx\n", param_base);
memset(params, 0, sizeof(struct param_struct));
//填写params的两个成员
//Linux kernel采用了页表方式，设置页表的大小，这里是4K
params->u1.s.page_size = LINUX_PAGE_SIZE;
params->u1.s.nr_pages = (DRAM_SIZE >> LINUX_PAGE_SHIFT);
/* set linux command line */
linux_cmd = get_linux_cmd_line();
if (linux_cmd == NULL) {
printk("Wrong magic: could not found linux command line\n");
} else {
//把命令行参数复制到params的commandline成员
memcpy(params->commandline, linux_cmd, strlen(linux_cmd) + 1);
printk("linux command line is: \"%s\"\n", linux_cmd);
}
}
如上，把不相关部分去掉了，加了注释。可以看出，这里就设置了param_struct必需的三个成员，核心是commandline。关于param_struct在linux内核的【include/arm/setup.h】中，(非全文)
