using device-tree fragments/overlays

[Tim Harvey]

Greetings,

I'm working on some features for the imx8mm-venice boards I support
which depend on making small changes to the dt before booting Linux.

The purpose is not to have any of this apply to the U-Boot controlling
dt but instead to the Linux kernel dt applied within ft_board_setup. I
could apply these changes with code but it would be way more readable
if there was a way to store these as fragments in the FIT image and
apply them.

The fragments would be small and have to do with two UART's routed to
a multi-protocol RS232/RS485 transceiver that can be used as either of
the following:
 - two tx/rx UART's (the default configuration for my dt's)
 - one rs232 uart with CTS/RTS flow control
 - one rs485 uart

Here would be an example dt overlay to apply over the default
arch/arm/dts/imx8mm-venice-gw73xx-0x.dts to change it from 2x RS232
UARTS with TX/RX (uart2/uart4) to RS485 half duplex:

/* For RS485 HD:
 *  - UART4_TX is DE for RS485 transmitter
 *  - RS485_EN needs to be pulled high
 *  - RS485_HALF needs to be pulled high
 *  - RS485_TERM enables on-chip termination
 */
&uart2 {
        rts-gpios = <&gpio5 29 GPIO_ACTIVE_HIGH>;
        rs485-term-gpios = <&gpio1 0 GPIO_ACTIVE_HIGH>;
};

&uart4 {
        status = "disabled";
};

&iomuxc {
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_hog>;

        pinctrl_hog: hoggrp {
                fsl,pins = <
                        MX8MM_IOMUXC_GPIO1_IO00_GPIO1_IO0
0x40000104 /* RS485_TERM */
                        MX8MM_IOMUXC_SAI1_RXFS_GPIO4_IO0
0x40000144 /* RS485_EN */
                        MX8MM_IOMUXC_SAI1_RXD0_GPIO4_IO2
0x40000144 /* RS485_HALF */
                >;
        };

        pinctrl_uart2: uart2grp {
                fsl,pins = <
                        MX8MM_IOMUXC_UART2_RXD_UART2_DCE_RX     0x140
                        MX8MM_IOMUXC_UART2_TXD_UART2_DCE_TX     0x140
                        MX8MM_IOMUXC_UART4_TXD_GPIO5_IO29       0x140
                >;
        };
};

Anyone do anything like this before or work with dt fragments/overlays
in U-Boot or have any other suggestions?

Best regards,

Tim