Uboot passes arguments to kernel

u-boot 要傳參數到kernel可使用 setenv bootargs
cmd line sample
# setenv bootargs androidboot.selinux=disable
# saveenv

code sample
setenv("bootargs", "androidboot.selinux=disable");

但是在android時, 會在image-android.c中的android_image_get_kernel讀取boot.img中的bootargs
boot.img的bootargs放在 BOARD_KERNEL_CMDLINE
BOARD_KERNEL_CMDLINE := console=ttymxc4,115200 init=/init

需注意的是如果code當中有設bootargs的話, 那在android_image_get_kernel就不會使用BOARD_KERNEL_CMDLINE設定的bootargs

 
int len = 0;
if (*hdr->cmdline) {
 len += strlen(hdr->cmdline);
}

char *bootargs = getenv("bootargs");
if (bootargs)
 len += strlen(bootargs);

char *newbootargs = malloc(len + 2);
if (!newbootargs) {
 puts("Error: malloc in android_image_get_kernel failed!\n");
 return -ENOMEM;
}
*newbootargs = '\0';

if (bootargs) {
 strcpy(newbootargs, bootargs);
} else if (*hdr->cmdline) {
 strcat(newbootargs, hdr->cmdline);
}

接著bootargs會傳到proc/cmdline, 可使用cat來確認傳入的parameter.
若要再交給android做為properties使用, 那可以在bootargs增加androidboot.xxx=xxx
在system/core/init/init.c當中會去處理bootargs.

static void process_kernel_cmdline(void)
{
    /* don't expose the raw commandline to nonpriv processes */

    chmod("/proc/cmdline", 0440);

    /* first pass does the common stuff, and finds if we are in qemu.
     * second pass is only necessary for qemu to export all kernel params
     * as props.
     */

    import_kernel_cmdline(0, import_kernel_nv); //read kernel parameter here

    if (qemu[0])
        import_kernel_cmdline(1, import_kernel_nv);

    /* now propogate the info given on command line to internal variables
     * used by init as well as the current required properties
     */
    export_kernel_boot_props();
}

void import_kernel_cmdline(int in_qemu,
                           void (*import_kernel_nv)(char *name, int in_qemu))
{
    char cmdline[2048];
    char *ptr;
    int fd;

    fd = open("/proc/cmdline", O_RDONLY);

    if (fd >= 0) {
        int n = read(fd, cmdline, sizeof(cmdline) - 1);
        if (n < 0) n = 0;
        /* get rid of trailing newline, it happens */
        if (n > 0 && cmdline[n-1] == '\n') n--;
        cmdline[n] = 0;

        close(fd);

    } else {
        cmdline[0] = 0;
    }

    ptr = cmdline;
    while (ptr && *ptr) {
        char *x = strchr(ptr, ' ');
        if (x != 0) *x++ = 0;
        import_kernel_nv(ptr, in_qemu);

        ptr = x;
    }
}

在import_kernel_nv當中會將androidboot.xxx都換成ro.boot.xxx

static void import_kernel_nv(char *name, int for_emulator)
{
    char *value = strchr(name, '=');
    int name_len = strlen(name);

    if (value == 0) return;
    *value++ = 0;

    if (name_len == 0) return;

    if (for_emulator) {
        /* in the emulator, export any kernel option with the
         * ro.kernel. prefix */
        char buff[PROP_NAME_MAX];
        int len = snprintf( buff, sizeof(buff), "ro.kernel.%s", name );

        if (len < (int)sizeof(buff))
            property_set( buff, value );

        return;
    }

    if (!strcmp(name,"qemu")) {
        strlcpy(qemu, value, sizeof(qemu));
    } else if (!strncmp(name, "androidboot.", 12) && name_len > 12) {
        const char *boot_prop_name = name + 12;
        char prop[PROP_NAME_MAX];
        int cnt;

        cnt = snprintf(prop, sizeof(prop), "ro.boot.%s", boot_prop_name); //change to ro.boot.xxx here

        if (cnt < PROP_NAME_MAX)
            property_set(prop, value);

    }
}

所以如果一開始有設置androidboot.selinux=disable的話, 在經過這邊之後會被換成ro.boot.selinux=disable, boot進android之後使用getprop就可以看到了.

另外一些特定的properties則會經過export_kernel_boot_props再parsing一次.

    struct {
        const char *src_prop;
        const char *dest_prop;
        const char *def_val;
    } prop_map[] = {
        { "ro.boot.serialno", "ro.serialno", "", },
        { "ro.boot.mode", "ro.bootmode", "unknown", },
        { "ro.boot.baseband", "ro.baseband", "unknown", },
        { "ro.boot.bootloader", "ro.bootloader", "unknown", },
    };