OpenWrt NetSoM developers guide. Adding support of microSD card.

Here explained steps of SDHC interface usage to get on OpenWrt MicroSD card.

Schematic

First of all we need to check schematic to determine what pads used for microSD:

There is connection to MicroSD socket.

Following Microprocessor pads used:

DTS

Next step is modification of DTS to be related to schematic.

Root node presenting each interface:

usdhc1 = &usdhc1;

Node implementation:

&usdhc1 {
pinctrl-names = «default»;
pinctrl-0 = <&pinctrl_usdhc1>;
cd-gpios = <&gpio1 19 GPIO_ACTIVE_LOW>;
keep-power-in-suspend;
wakeup-source;
status = «okay»;
}

And the last one is map of PADS used by interface:

pinctrl_usdhc1: usdhc1grp {
fsl,pins = <
MX6UL_PAD_SD1_CMD__USDHC1_CMD              0x170b9
MX6UL_PAD_SD1_CLK__USDHC1_CLK                  0x100b9
MX6UL_PAD_SD1_DATA0__USDHC1_DATA0       0x170b9
MX6UL_PAD_SD1_DATA1__USDHC1_DATA1       0x170b9
MX6UL_PAD_SD1_DATA2__USDHC1_DATA2       0x170b9
MX6UL_PAD_SD1_DATA3__USDHC1_DATA3       0x170b9
>;
};

Some explanation of ways for DTS modification may be found here. In short — there are lot of gaps in Linux documentation so you have to read source code to learn the Linux. For example, here is commit that moves SDHC interface used by MicroSD from one set of PADS to another.

After DTS modification it is time to rebuild image and update firmware. Explained here. The only last step after that is mount microSD card after your system boots with new image:

# ls /dev/mmc*
/dev/mmcblk0          /dev/mmcblk0p1

Here we see microSD card with one partition.

Mount it:

# mount      /dev/mmcblk0p1      /mnt/

And check:

# ls /mnt/
hello.txt            lost+found

Here is previously prepared MicroSD card (EXT4 formatted).