Linux kernel 版本降级

## 前言

今天遇到个无奈的问题。有个加解密工具不支持kernel 5.15，需要降级到5.4.135

## 开始降级

1. 安装
```shell
sudo apt update
sudo apt install linux-image-5.4-generic linux-headers-5.4-generic
sudo update-grub
```

2. 修改/etc/default/grub文件
```shell
sudo vim /etc/default/grub
```

```shell
	# If you change this file, run 'update-grub' afterwards to update
	# /boot/grub/grub.cfg.
	# For full documentation of the options in this file, see:
	#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
	GRUB_TIMEOUT_STYLE=hidden
	GRUB_TIMEOUT=0
	GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
	GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
	GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
	# This works with Linux (no patch required) and with any kernel that obtains
	# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
	#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
	#GRUB_TERMINAL=console

# The resolution used on graphical terminal
	# note that you can use only modes which your graphic card supports via VBE
	# you can see them in real GRUB with the command `vbeinfo'
	#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
	#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
	#GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
	#GRUB_INIT_TUNE="480 440 1"
	```
修改两个参数`GRUB_TIMEOUT_STYLE`要将`hidden`改成`menu`。将`GRUB_TIMEOUT`设置成`5`
`GRUB_TIMEOUT_STYLE`：表示是否显示grub菜单，`hidden`是不显示，`menu`是显示
`GRUB_TIMEOUT`：grub菜单显示的时间长度，设置成5秒
设置完之后保存重启
```shell
reboot
```

3. 使用低版本开机
重启后，会进入grub选择菜单，高级选项中选择5.4的kernel就可以了。进入系统后，需要删除不用的高版本kernel，这样以后每次进入都是默认5.4的kernel
```shell
 dpkg -l | grep linux-image  #列出现有kernel
```
使用sudo apt remove 命令将不要的kernel删除，大工就搞成了