Re: using device-tree fragments/overlays

14 Oct 2021

      On Wed, Oct 13, 2021 at 1:38 AM Peter Robinson pbrobinson@gmail.com wrote:
...
Hi Tim,
...
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?
A feature for overlays landed upstream in May [1], so the 2021,07
release, so I'm sure there's most of the functionality you require
already upstream. Not sure how a user would specify which of the
variants to use or how it would load it from the FIT but I'm sure the
patch series provides you a good start point.
Peter
[1] https://lists.denx.de/pipermail/u-boot/2021-May/448794.html
Peter,
Thanks - this is very helpful and is what I was looking for. My plan
is to use the hwconfig env var for users to select how they want
serial configured. I can use the methods used in the extension_apply
function and it seems appropriate for me to put dtbo's on the root
filesystem with the dtb's when using this.
Best regards,
Tim

Re: using device-tree fragments/overlays

Tim Harvey