树莓派 4b 如何配置 2 个串口？

我现在是用的 jlink+openocd 来调试的

在`/boot/config.txt`里面打开就行

@LXGMAX 感谢回复，用树莓派官方的 os 和 fedora 官方的 os 确实是可以，但我用不了。

首先我不用树莓派官方的 os ，其次我用的 fedora 的内核是直接让 u-boot 或让树莓派自己的固件去加载的，而不是 fedora 官方镜像那种 u-boot 加载 grub ，再让 grub 加载的，fedora 的官方镜像启动一次要好几分钟，我自己编译的内核不用 initramfs ，自己写 init 脚本，从上电到能 ssh 登录就 11 秒，所以我不想用 fedora 官方镜像，想找一个办法在我目前的系统上启用其它的串口。

就这一个问题我整了 2 天没搞定，难受死了，树莓派性能太差了，稍微修改一下再重试都要等好久

@wniming 内核改下 dts 参考： https://elixir.bootlin.com/linux/latest/source/arch/arm/boot/dts/bcm2711-rpi-4-b.dts

然后加入
```
&uart3 {
status = "okay";
};
```
要用哪个就启用哪个

@LXGMAX 感谢回复，内核的这个 dts 实际是启动不了树莓派 4b 的，u-boot 官方的树莓派 4b 的 dts 也是实际启动不了树莓派 4b 的，我现在只呢用从树莓派 os 系统镜像里提取的 dts 来启动，所以修改内核的 dts 这种方法我是用不了的。

@LXGMAX
如果我直接用 mainline 内核的 dts 编译出来的 dtb 就是这种报错：

DRAM: 3.9 GiB
RPI 4 Model B (0xc03115)
Core: 175 devices, 17 uclasses, devicetree: board
MMC: mmc@7e300000: 0, mmc@7e340000: 1
Loading Environment from FAT... Card did not respond to voltage select! : -110
** Bad device specification mmc 0 **
In: serial
Out: vidconsole
Err: vidconsole
Net: eth0: ethernet@7d580000
PCIe BRCM: link up, 5.0 Gbps x1 (SSC)
starting USB...
Bus xhci_pci: Register 5000420 NbrPorts 5
Starting the controller
USB XHCI 1.00
scanning bus xhci_pci for devices... 4 USB Device(s) found
scanning usb for storage devices... 1 Storage Device(s) found
Hit any key to stop autoboot: 0
Card did not respond to voltage select! : -110
switch to partitions #0, OK
mmc1 is current device
Scanning mmc 1:1...
Found U-Boot script /boot.scr
493 bytes read in 6 ms (80.1 KiB/s)
## Executing script at 02400000
Card did not respond to voltage select! : -110
** Bad device specification mmc 0 **
Couldn't find partition mmc 0:1
Can't set block device
46483968 bytes read in 144 ms (307.9 MiB/s)
17868889 bytes read in 56 ms (304.3 MiB/s)
ERROR: Did not find a cmdline Flattened Device Tree
Could not find a valid device tree
SCRIPT FAILED: continuing...
MMC Device 2 not found
no mmc device at slot 2

Device 0: Vendor: ZX1 256G Rev: U091 Prod:
Type: Hard Disk
Capacity: 244198.3 MB = 238.4 GB (500118192 x 512)
... is now current device
Scanning usb 0:2...
BOOTP broadcast 1
BOOTP broadcast 2
BOOTP broadcast 3
BOOTP broadcast 4
DHCP client bound to address 192.168.2.150 (3010 ms)
*** Warning: no boot file name; using 'C0A80296.img'
Using ethernet@7d580000 device
TFTP from server 192.168.2.1; our IP address is 192.168.2.150
Filename 'C0A80296.img'.
Load address: 0x1000000
Loading: T T T T T T T T T T
Retry count exceeded; starting again
missing environment variable: pxeuuid
Retrieving file: pxelinux.cfg/01-e4-5f-01-a8-b0-00
Using ethernet@7d580000 device
TFTP from server 192.168.2.1; our IP address is 192.168.2.150
Filename 'pxelinux.cfg/01-e4-5f-01-a8-b0-00'.
Load address: 0x2500000
Loading: *

@LXGMAX 我在 u-boot 的脚本里通过 fdt 的 apply 命令把 uart 相关的 dtbo 打上去就能开启第 2 个串口了。