在嵌入式Linux开发中，先运行的是u-boot，然后再加载Linux内核。
启动时如果设置了u-boot等待时间，在等待时间内，按任意键就会进入u-boot命令行环境。
在u-boot命令行环境下，可以使用gpio命令来操作GPIO。
Synopsis
gpio <input|set|clear|toggle> <pin>
gpio read <name> <pin>
gpio status [-a] [<bank>|<pin>]
gpio 命令用于访问通用输入/输出。
The gpio command is used to access General Purpose Inputs/Outputs.
gpio input
将 GPIO 引脚切换为输入模式。
Switch the GPIO pin to input mode.
gpio set
将 GPIO 引脚切换至输出模式，并将信号设置为 1。
Switch the GPIO pin to output mode and set the signal to 1.
gpio clear
将 GPIO 引脚切换至输出模式，并将信号设置为 0。
Switch the GPIO pin to output mode and set the signal to 0.
gpio toggle
将 GPIO 引脚切换至输出模式，并反转信号状态。
Switch the GPIO pin to output mode and reverse the signal state.
gpio read
读取 GPIO 引脚的信号状态，并将其保存在环境变量名称中。
Read the signal state of the GPIO pin and save it in environment variable name.
gpio status
显示一个或多个 GPIO 的状态。默认情况下，只显示声称的 GPIO。 gpio status 命令的输出字段有：
Display the status of one or multiple GPIOs. By default only claimed GPIOs are displayed. gpio status command output fields are:
<name>: <function>: <value> [x] <label>
函数可以取以下值:
function can take the following values:
output
pin configured in gpio output, value indicates the pin’s level
input
pin configured in gpio input, value indicates the pin’s level
func
pin configured in alternate function, followed by label which shows pinmuxing label.
unused
pin not configured
[x] or [ ] indicate respectively if the gpio is used or not.
label shows the gpio label.
Parameters
-a
Display GPIOs irrespective of being claimed. 显示 GPIO，无论其是否被申领。
bank
Name of a bank of GPIOs to be displayed. 显示 GPIO 组的名称。
pin
Name of a single GPIO to be displayed or manipulated.要显示或操作的单个 GPIO 的名称。
Examples
Switch the status of a GPIO:
=> gpio set a5
gpio: pin a5 (gpio 133) value is 1
=> gpio clear a5
gpio: pin a5 (gpio 133) value is 0
=> gpio toggle a5
gpio: pin a5 (gpio 133) value is 1
=> gpio read myvar a5
gpio: pin a5 (gpio 133) value is 1
=> echo $myvar
1
=> gpio toggle a5
gpio: pin a5 (gpio 133) value is 0
=> gpio read myvar a5
gpio: pin a5 (gpio 133) value is 0
=> echo $myvar
0
Show the GPIO status:
=> gpio status
Bank GPIOA:
GPIOA1: func rgmii-0
GPIOA2: func rgmii-0
GPIOA7: func rgmii-0
GPIOA10: output: 0 [x] hdmi-transmitter@39.reset-gpios
GPIOA13: output: 1 [x] red.gpios
Bank GPIOB:
GPIOB0: func rgmii-0
GPIOB1: func rgmii-0
GPIOB2: func uart4-0
GPIOB7: input: 0 [x] mmc@58005000.cd-gpios
GPIOB11: func rgmii-0
Configuration
gpio 命令只有在 CONFIG_CMD_GPIO=y 时可用。gpio 读取命令只有在 CONFIG_CMD_GPIO_READ=y 时可用。
The gpio command is only available if CONFIG_CMD_GPIO=y. The gpio read command is only available if CONFIG_CMD_GPIO_READ=y.
Return value
如果命令成功执行，返回值 $? 设置为 0；如果命令出错，返回值 $? 设置为 1。
If the command succeds the return value $? is set to 0. If an error occurs, the return value $? is set to 1.
我的u-boot版本和构建信息：
u-boot=> help
...
version - print monitor, compiler and linker version
...
u-boot=> version
U-Boot 2023.04-lf_v2023.04+g49b102d988 (Nov 21 2023 - 07:28:53 +0000)
aarch64-poky-linux-gcc (GCC) 12.3.0
GNU ld (GNU Binutils) 2.40.0.20230703
u-boot=> gpio status -a
Bank GPIO2_:
GPIO2_0: input: 1 [ ]
GPIO2_1: output: 0 [ ]
GPIO2_2: input: 0 [ ]
GPIO2_3: input: 0 [ ]
GPIO2_4: input: 0 [ ]
GPIO2_5: input: 0 [ ]
GPIO2_6: input: 0 [ ]
GPIO2_7: input: 0 [ ]
GPIO2_8: input: 0 [ ]
GPIO2_9: input: 0 [ ]
GPIO2_10: input: 0 [ ]
GPIO2_11: output: 0 [ ]
GPIO2_12: input: 0 [ ]
GPIO2_13: input: 1 [ ]
GPIO2_14: input: 0 [ ]
GPIO2_15: input: 0 [ ]
GPIO2_16: input: 0 [ ]
GPIO2_17: input: 0 [ ]
GPIO2_18: input: 0 [ ]
GPIO2_19: input: 0 [ ]
GPIO2_20: input: 0 [ ]
GPIO2_21: input: 0 [ ]
GPIO2_22: input: 1 [ ]
GPIO2_23: input: 0 [ ]
GPIO2_24: input: 0 [ ]
GPIO2_25: input: 0 [ ]
GPIO2_26: input: 0 [ ]
GPIO2_27: input: 0 [ ]
GPIO2_28: input: 1 [ ]
GPIO2_29: input: 0 [ ]
GPIO2_30: input: 0 [ ]
GPIO2_31: input: 0 [ ]
Bank GPIO3_:
GPIO3_0: input: 1 [x] mmc@42860000.cd-gpios
GPIO3_1: input: 0 [ ]
GPIO3_2: input: 0 [ ]
GPIO3_3: input: 0 [ ]
GPIO3_4: input: 0 [ ]
GPIO3_5: input: 0 [ ]
GPIO3_6: input: 0 [ ]
GPIO3_7: input: 0 [ ]
GPIO3_8: input: 0 [ ]
GPIO3_9: input: 0 [ ]
GPIO3_10: input: 0 [ ]
GPIO3_11: input: 0 [ ]
GPIO3_12: input: 0 [ ]
GPIO3_13: input: 0 [ ]
GPIO3_14: input: 0 [ ]
GPIO3_15: input: 0 [ ]
GPIO3_16: input: 0 [ ]
GPIO3_17: input: 0 [ ]
GPIO3_18: input: 0 [ ]
GPIO3_19: input: 0 [ ]
GPIO3_20: input: 0 [ ]
GPIO3_21: input: 0 [ ]
GPIO3_22: input: 0 [ ]
GPIO3_23: input: 0 [ ]
GPIO3_24: input: 0 [ ]
GPIO3_25: input: 0 [ ]
GPIO3_26: input: 0 [ ]
GPIO3_27: input: 0 [ ]
GPIO3_28: input: 0 [ ]
GPIO3_29: input: 0 [ ]
GPIO3_30: input: 0 [ ]
GPIO3_31: input: 0 [ ]
Bank GPIO4_:
GPIO4_0: input: 0 [ ]
GPIO4_1: input: 0 [ ]
GPIO4_2: input: 0 [ ]
GPIO4_3: input: 0 [ ]
GPIO4_4: input: 0 [ ]
GPIO4_5: input: 0 [ ]
GPIO4_6: input: 0 [ ]
GPIO4_7: input: 0 [ ]
GPIO4_8: input: 0 [ ]
GPIO4_9: input: 0 [ ]
GPIO4_10: input: 0 [ ]
GPIO4_11: input: 0 [ ]
GPIO4_12: input: 0 [ ]
GPIO4_13: input: 0 [ ]
GPIO4_14: input: 0 [ ]
GPIO4_15: input: 0 [ ]
GPIO4_16: input: 0 [ ]
GPIO4_17: input: 1 [ ]
GPIO4_18: input: 0 [ ]
GPIO4_19: input: 0 [ ]
GPIO4_20: input: 0 [ ]
GPIO4_21: input: 0 [ ]
GPIO4_22: input: 0 [ ]
GPIO4_23: input: 0 [ ]
GPIO4_24: input: 1 [ ]
GPIO4_25: input: 0 [ ]
GPIO4_26: input: 0 [ ]
GPIO4_27: input: 0 [ ]
GPIO4_28: input: 0 [ ]
GPIO4_29: input: 0 [ ]
GPIO4_30: input: 0 [ ]
GPIO4_31: input: 0 [ ]
Bank GPIO1_:
GPIO1_0: input: 1 [ ]
GPIO1_1: input: 1 [ ]
GPIO1_2: input: 0 [ ]
GPIO1_3: input: 0 [ ]
GPIO1_4: input: 0 [ ]
GPIO1_5: input: 0 [ ]
GPIO1_6: input: 0 [ ]
GPIO1_7: input: 1 [ ]
GPIO1_8: input: 0 [ ]
GPIO1_9: input: 0 [ ]
GPIO1_10: input: 0 [ ]
GPIO1_11: input: 0 [ ]
GPIO1_12: input: 0 [ ]
GPIO1_13: input: 0 [ ]
GPIO1_14: input: 0 [ ]
GPIO1_15: input: 0 [ ]
GPIO1_16: input: 0 [ ]
GPIO1_17: input: 0 [ ]
GPIO1_18: input: 0 [ ]
GPIO1_19: input: 0 [ ]
GPIO1_20: input: 0 [ ]
GPIO1_21: input: 0 [ ]
GPIO1_22: input: 0 [ ]
GPIO1_23: input: 0 [ ]
GPIO1_24: input: 0 [ ]
GPIO1_25: input: 0 [ ]
GPIO1_26: input: 0 [ ]
GPIO1_27: input: 0 [ ]
GPIO1_28: input: 0 [ ]
GPIO1_29: input: 0 [ ]
GPIO1_30: input: 0 [ ]
GPIO1_31: input: 0 [ ]
GPIO bank和pin name可以参照status的结果，然后运行gpio命令：
u-boot=> gpio status GPIO3_
Bank GPIO3_:
GPIO3_0: input: 1 [x] mmc@42860000.cd-gpios
u-boot=> gpio status GPIO3_0
Bank GPIO3_:
GPIO3_0: input: 1 [x] mmc@42860000.cd-gpios
u-boot=> gpio status GPIO3_1
Bank GPIO3_:
GPIO3_1: input: 0 [ ]
u-boot=> gpio set GPIO2_11
gpio: pin GPIO2_11 (gpio 11) value is 1
u-boot=> gpio clear GPIO2_11
gpio: pin GPIO2_11 (gpio 11) value is 0
参考：
