如何在 ARM 上设置 Linux 内核命令行? [英] How to set Linux kernel command line on ARM?

问题描述

我的理解是，对于 ARM，内核引导命令行一般有三个来源:

My understanding is that for ARM there are three sources for the kernel boot command line in general:

1. 在内核配置中作为 CONFIG_CMDLINE 给出的那些
2. 引导加载程序传递的那些(通常是 ARM 处理器上的 U-Boot)
3. 那些包含在设备树中的，在 selected/bootargs 下使用哪一个取决于内核配置参数.我的问题是如何使用内核配置在这些选项之间进行选择?

一个可以附加到另一个，即我们可以使用 CONFIG_CMDLINE 传递一些，然后在设备树中附加硬件特定的参数吗?

And can one append to another i.e. can we pass some using CONFIG_CMDLINE and then append hardware specific parameters in device tree?

我正在尝试组合 1、2 和 3，但这不能编译:

I'm trying combination 1, 2 AND 3 to begin with but this doesn't compile:

/dts-v1/; 
#include "imx6q.dtsi"
#include "imx6q-h.dtsi"
#include "imx6q-m.dtsi"
/ {
    model = "A M";
    compatible = "a,imx6q-hydra13", "a,imx6q-mercury",
                    "a,imx6q-hydra", "fsl,imx6q";
};

&ssd_touch {
    status = "okay";
};

ERROR AT THIS LINE: chosen {  
        bootargs = "console=ttymxc1,115200";
};

推荐答案

我的理解是，对于 ARM，内核引导命令行一般有三个来源:

My understanding is that for ARM there are three sources for the kernel boot command line in general:

这对于 Linux ARM 内核来说并不准确.内核只处理两个源"，一个 default kernel command 字符串和一个 bootloader 内核参数 字符串.
更多详情如下.

That's not accurate for the Linux ARM kernel. The kernel only deals with two "sources", a default kernel command string and a bootloader kernel arguments string.
More details follow.

我的问题是如何使用内核配置在这些选项之间进行选择?

My question is how to choose between these options using kernel configuration?

您的选择可能仅限于使用内核配置".

附加"命令行配置选项，即 CONFIG_CMDLINE_FROM_BOOTLOADER(使用 bootloader 内核参数，如果可用")、CONFIG_CMDLINE_EXTEND(扩展 bootloader 内核参数")和 CONFIG_CMDLINE_FORCE(";始终使用 default kernel command string") 仅在启用对旧 ATAG 参数传递(即 CONFIG_ATAGS)的支持时才可用(自 3.7 版起).
然而 CONFIG_ATAGS 确实默认为 y 除非明确禁用.主线 arch/arm/configs/ 中的十几个 _defconfig 文件明确禁用了这个 CONFIG_ATAGS.

The "additional" command-line configuration choices, i.e. CONFIG_CMDLINE_FROM_BOOTLOADER ("Use bootloader kernel arguments if available"), CONFIG_CMDLINE_EXTEND ("Extend bootloader kernel arguments"), and CONFIG_CMDLINE_FORCE ("Always use the default kernel command string") are only available (since version 3.7) when support for the old ATAGs parameter passing (i.e. CONFIG_ATAGS) is enabled.
However CONFIG_ATAGS does default to y unless explicitly disabled. About a dozen _defconfig files in mainline arch/arm/configs/ do explicitly disable this CONFIG_ATAGS.

但是设备树在哪里适合这种方案?

But where does the device tree fit in this scheme of things?

设备树是引导加载程序内核参数的提供者.
也就是说，/chosen 节点中的 bootargs= 属性，是向 ARM 内核提供命令行的常规方法，即当 CONFIG_ATAGS 被禁用时，或者 CONFIG_CMDLINE_FROM_BOOTLOADER或 CONFIG_CMDLINE_EXTEND 已启用.
命令行由内核从设备树中作为字符串通过 early_init_dt_scan_chosen() 在 drivers/of/fdt.c

The Device Tree is the provider of bootloader kernel arguments.
That is, the bootargs= property in the /chosen node, is the conventional method of providing the command line to the ARM kernel, i.e. when CONFIG_ATAGS is disabled, or either CONFIG_CMDLINE_FROM_BOOTLOADER or CONFIG_CMDLINE_EXTEND are enabled.
The command line is retrieved by the kernel from the Device Tree as a string by early_init_dt_scan_chosen() in drivers/of/fdt.c

仅当启用 CONFIG_CMDLINE_FORCE(带有 CONFIG_ATAGS)时，设备树 bootargs= 属性才会被忽略.

Only if CONFIG_CMDLINE_FORCE (with CONFIG_ATAGS) are enabled will the Device Tree bootargs= property be ignored.

您可以使用 CONFIG_CMDLINE 使用 默认内核命令 配置/构建 ARM 内核，以防其他方法无法设置命令行.
drivers/of/fdt.c 中的注释记录了这一点.

You can configure/build the ARM kernel with a default kernel command using CONFIG_CMDLINE in case nothing else managed to set the command line.
A comment in drivers/of/fdt.c documents this.

CONFIG_CMDLINE_EXTEND(带 CONFIG_ATAGS)生成一个命令行，它是设备树 bootargs= 属性与 CONFIG_CMDLINE 内容的串联.

CONFIG_CMDLINE_EXTEND (with CONFIG_ATAGS) results in a command line that is the concatenation of the Device Tree bootargs= property with the contents of CONFIG_CMDLINE.

不过……

当使用 U-Boot 引导 Linux 内核时，请注意当环境变量 bootargs 被定义时，U-Boot 将(尝试)安装那个 bootargs环境变量(作为属性)添加到加载的设备树 blob 的 /chosen 节点中.
如果/chosen节点在DT中不存在，则创建.
如果该 DT 节点中不存在 bootargs= 属性，则创建它.
如果 bootargs= 属性已存在于该 DT 节点中，则它会被 U-Boot 环境变量覆盖.
请参阅 common/fdt_support.c 中的 fdt_chosen().

When using U-Boot to boot the Linux kernel, be aware that when the environment variable bootargs is defined, U-Boot will (try to) install that bootargs environment variable (as a property) into the the /chosen node of the loaded Device Tree blob.
If the /chosen node does not exist in the DT, then it is created.
If the bootargs= property does not exist in that DT node, then it is created.
If the bootargs= property already exists in that DT node, then it is overwritten with the U-Boot environment variable.
See fdt_chosen() in common/fdt_support.c.

IOW U-Boot 的 bootargs 环境变量通常成为事实上的内核命令行.

IOW U-Boot's bootargs environment variable typically becomes the de facto kernel command line.

一个可以附加到另一个，即我们可以使用 CONFIG_CMDLINE 传递一些，然后在设备树中附加硬件特定的参数吗?

And can one append to another i.e. can we pass some using CONFIG_CMDLINE and then append hardware specific parameters in device tree?

仅当 (a) CONFIG_ATAGS 已启用，并且 (b) CONFIG_CMDLINE_EXTEND 已启用，并且 (c) 确保 U-Boot 的环境中没有 bootargs 变量.

Only if (a) CONFIG_ATAGS is enabled, and (b) CONFIG_CMDLINE_EXTEND is enabled, and (c) ensure that there is no bootargs variable in U-Boot's environment.

底线

1. U-Boot 总是尝试使用设备树将其 bootargs 变量传递给内核.
2. 设备树中的 bootargs= 属性始终被内核用作 bootloader 内核参数，如 arch/arm/Kconfig 中所述强> 文件.

1. U-Boot always tries to pass its bootargs variable to the kernel using the Device Tree.
2. The bootargs= property in the Device Tree is always used by the kernel as the bootloader kernel arguments as mentioned in the arch/arm/Kconfig file.

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