Modify below dts files
l4t/sources/hardware/nvidia/soc/t19x/kernel-dts/tegra194-soc/tegra194-soc-sdhci.dtsi

sdmmc3: sdhci@3440000 —> external sd card

sdmmc1: sdhci@3400000 —> dev board sd card

hardware/nvidia/platform/t19x/jakku/kernel-dts/common/tegra194-fixed-regulator-p3668.dtsi

             p3668_vdd_sdmmc3_sw: regulator@108 {

                        compatible = "regulator-fixed";

                        reg = <108>;

                        regulator-name = "vdd-sdmmc3-sw";

                        regulator-min-microvolt = <3300000>;

                        regulator-max-microvolt = <3300000>;

                        gpio = <&tegra_main_gpio TEGRA194_MAIN_GPIO(H, 1) 0>;

                        enable-active-high;

                };

How to decide to use 108?

solution: pick up one unused reg.

cd  Jetson/l4t/sources/kernel/kernel-4.9/output/arch/arm64/boot/dts

sudo ../../../../scripts/dtc/dtc -I dtb -O dts -o output.txt tegra194-p3668-all-xhiveai-devkit.dtb

vi output.txt

fixed-regulators {

regulator@0

regulator@106

regulator@107

regulator@104

regulator@101

regulator@102

regulator@105

regulator@111

regulator@112

regulator@113

regulator@200

}

find out the unused reg: 108

hardware/nvidia/platform/t19x/jakku/kernel-dts/common/tegra194-p3668-xhiveai-common.dtsi

        /*sdmmc3: external sd card*/

        sdmmc3: sdhci@3440000 {

                mmc-ocr-mask = <0x0>;

                cd-inverted;

                cd-gpios = <&tegra_main_gpio TEGRA194_MAIN_GPIO(Q, 2) 0>;

                nvidia,cd-wakeup-capable;

                mmc-ocr-mask = <0>;

                cd-inverted;

                vmmc-supply = <&p3668_vdd_sdmmc3_sw>;

                status = "okay";

        };

hardware/nvidia/platform/t19x/jakku/kernel-dts/common/tegra194-power-tree-p3668.dtsi 

      sdhci@3440000 {

                vmmc-supply = <&p3668_vdd_sdmmc3_sw>;

        };

Check the sdhci status
cd /proc/device-tree/

cd sdhci@3440000/

cat status

the device generated: /dev/mmcblk1