高通MDM平台-LK

1、前言

这篇文章介绍了MDM平台的Little Kernel（LK）的启动流程。Little Kernel的作用是在启动的时候初始化硬件，从存储器中载入Linux内核和ramdisk到RAM中，配置初始化寄存器和命令行参数，最后跳转到内核中运行

2、LK

2.1代码路径

LK的代码在apps_proc\bootable\bootloader\lk目录

2.2代码概述

从C程序入口开始讲起kmain

target_init：初始化flash参数，以及分区表

qpic_nand_init(&config);//初始化flash 参数，如page size等ptable_init(&flash_ptable);smem_ptable_init();//读取分区表smem_add_modem_partitions(&flash_ptable);update_ptable_names();flash_set_ptable(&flash_ptable);

支持的flash型号在supported_flash 定义，通过flash id索引，配置了pagesize 等参数

apps_init：加载kernel到ram，并配置命令行，跳转到kernel

调用 aboot_init，boot_into_fastboot boot_into_recovery flag确认进入fastboot 模式还是recovery 模式还是正常启动

check_reboot_mode 检查重启原因，查看是否是升级或者其他需要进入recovery模式的操作

unsigned check_reboot_mode(void)
{uint32_t restart_reason = 0;/* Read reboot reason and scrub it */restart_reason = readl(RESTART_REASON_ADDR);//读取寄存器值writel(0x00, RESTART_REASON_ADDR);return restart_reason;
}

该寄存器值在重启时调用sys_reboot （ apps_proc\system\core\powerapp/powerapp.c）中写该寄存器，标明是否需要进入recovery 模式

fastboot 模式

static void create_fastboot_mode()
{fastboot_register("oem xxx", cmd_oem_xxx);//定义OEM命令/* register aboot specific fastboot commands */aboot_fastboot_register_commands();初始化默认支持命令/* dump partition table for debug info */partition_dump();/* initialize and start fastboot */fastboot_init(target_get_scratch_address(), target_get_max_flash_size());打开usb通信，建立新的线程处理fastboot 命令
}

recovery模式正常启动

调用boot_system_normal(), 通过flag boot_into_recovery会决定传递给kernel的cmdline挂载哪个fs recoveryfs or system

boot_linux_from_flash()
{if(!boot_into_recovery) 通过flag确认 使用哪个kernel{ptn = ptable_find(ptable, "boot");if (ptn == NULL) {dprintf(CRITICAL, "ERROR: No boot partition found\n");return -1;}}else{ptn = ptable_find(ptable, "recovery");if (ptn == NULL) {dprintf(CRITICAL, "ERROR: No recovery partition found\n");return -1;}}flash_read(ptn, offset, buf, page_size) //读取 boot.img 的第一个page，也就是headerhdr->kernel_addr = VA((addr_t)(hdr->kernel_addr));hdr->ramdisk_addr = VA((addr_t)(hdr->ramdisk_addr));hdr->tags_addr = VA((addr_t)(hdr->tags_addr));flash_read(ptn, offset, (void *)hdr->kernel_addr, kernel_actual)//加载kernel到ram中，地址为kernel_addrflash_read(ptn, offset + dt_entry.offset, (void *)hdr->tags_addr, dt_entry.size)//加载dts到ram中，地址为tags_addrboot_linux((void *)hdr->kernel_addr, (void *)hdr->tags_addr,(const char *)hdr->cmdline, board_machtype(),(void *)hdr->ramdisk_addr, hdr->ramdisk_size);//启动kernel
}

上面提到读取boot.img的header，在bootable/bootloader/lk/app/aboot/bootimg.h 定义，也包括了boot.img的构成

 struct boot_img_hdr
{unsigned char magic[BOOT_MAGIC_SIZE];unsigned kernel_size;  /* size in bytes */unsigned kernel_addr;  /* physical load addr */unsigned ramdisk_size; /* size in bytes */unsigned ramdisk_addr; /* physical load addr */unsigned second_size;  /* size in bytes */unsigned second_addr;  /* physical load addr */unsigned tags_addr;    /* physical addr for kernel tags */unsigned page_size;    /* flash page size we assume */unsigned dt_size;      /* device_tree in bytes */unsigned unused;    /* future expansion: should be 0 */unsigned char name[BOOT_NAME_SIZE]; /* asciiz product name */unsigned char cmdline[BOOT_ARGS_SIZE];unsigned id[8]; /* timestamp / checksum / sha1 / etc */
};

boot.img 构成
** +-----------------+
** | boot header     | 1 page
** +-----------------+
** | kernel          | n pages
** +-----------------+
** | ramdisk         | m pages
** +-----------------+
** | second stage    | o pages
** +-----------------+
** | device tree     | p pages
** +-----------------+

结合实际的image，可以分析出header的各个参数

magic：41 4E 44 52 4F 49 44 21

kernel size 00 3D 8F 30 kernel addr 08 00 80 00

ram size 0 ram addr 08 00 80 00

second size 0 second addr 08 f0 00 00

tag addr 81 E0 00 00 page size 08 00

dt size 07 68 00

boot_linux最后更新了cmdline，并刷新到dts中，跳转到kernel

final_cmdline = update_cmdline((const char*)cmdline);
update_device_tree((void *)tags,(const char *)final_cmdline, ramdisk, ramdisk_size);
entry(0, machtype, (unsigned*)tags_phys);