Device tree and passing parameter to serial driver from specyfic device overlay

Question

I would like to pass linux,rs485-enabled-at-boot-time; flag/parameter to serial port driver configuration for sc16is752 device defined by this device tree overlay (unmodified file from Raspbian):

/dts-v1/;
/plugin/;

/ {
    compatible = "brcm,bcm2835";

    fragment@0 {
        target = <&gpio>;
        __overlay__ {
            spi1_pins: spi1_pins {
                brcm,pins = <19 20 21>;
                brcm,function = <3>; /* alt4 */
            };

            spi1_cs_pins: spi1_cs_pins {
                brcm,pins = <18>;
                brcm,function = <1>; /* output */
            };
        };
    };

    fragment@1 {
        target = <&spi1>;
        frag1: __overlay__ {
            #address-cells = <1>;
            #size-cells = <0>;
            pinctrl-names = "default";
            pinctrl-0 = <&spi1_pins &spi1_cs_pins>;
            cs-gpios = <&gpio 18 1>;
            status = "okay";


            sc16is752: sc16is752@0 {
                compatible = "nxp,sc16is752";
                reg = <0>; /* CE0 */
                clocks = <&sc16is752_clk>;
                interrupt-parent = <&gpio>;
                interrupts = <24 2>; /* IRQ_TYPE_EDGE_FALLING */
                gpio-controller;
                gpio-cells = <2>;
                spi-max-frequency = <4000000>;

                sc16is752_clk: sc16is752_clk {
                    compatible = "fixed-clock";
                    #clock-cells = <0>;
                    clock-frequency = <14745600>;
                };
            };
        };
    };

    fragment@2 {
        target = <&aux>;
        __overlay__ {
            status = "okay";
        };
    };

    __overrides__ {
      int_pin = <&sc16is752>,"interrupts:0";
    };
};

SC16IS752 is a chip with few GPIO pins and two UART interfaces with hardware RS485 support, and Linux driver for SC16IS752 can enable it:

https://github.com/raspberrypi/linux/blob/04c8e47067d4873c584395e5cb260b4f170a99ea/drivers/tty/serial/sc16is7xx.c#L729 (see line 729).

Device tree bindings for SC16IS752 have no such parameter, so I guess I can't just add linux,rs485-enabled-at-boot-time; inside sc16is752: sc16is752@0 node.

How can I pass that?

Edit:

I have some progress (thanks @Philippos).

I copied original sc16is752-spi1 overlay source from RPi kernel sources (it was somewhere in linux/arch/arm/boot/dts/overlays/).

I have added linux,rs485-enabled-at-boot-time; to frag1 like this:

(...)

        frag1: __overlay__ {
            #address-cells = <1>;
            #size-cells = <0>;
            pinctrl-names = "default";
            pinctrl-0 = <&spi1_pins &spi1_cs_pins>;
            cs-gpios = <&gpio 18 1>;
            status = "okay";

            linux,rs485-enabled-at-boot-time; /* HERE */

(...)

I compiled it like this:

dtc -O dtb -o sc16is752-ses.dtbo -b 0 -@ sc16is752-spi1-overlay-edited.dts

I copied output file to /boot/overlays and I put this line dtoverlay=sc16is752-ses,int_pin=24 instead of dtoverlay=sc16is752-spi1,int_pin=24 (original overlay from Raspbian) in /boot/config.txt.

After reboot I checked sudo vcdbg log msg to see if my overlay was loaded:

017319.445: brfs: File read: 1810 bytes
017327.250: brfs: File read: /mfs/sd/overlays/sc16is752-ses.dtbo
017346.010: Loaded overlay 'sc16is752-ses'
017346.065: dtparam: int_pin=24
019273.102: Device tree loaded to 0x2eff9400 (size 0x6b27)

I noticed that:

• my crystal clock change worked (my baud rates were messed up and now it is fine), but I see no RTS signal on logic analyzer)
• RTS line is LOW (it was HIGH with original overlay), but it does not change when I'm sending data

Yesterday I have tried to enable this feature in C++ by changing port flags and it worked, so my hardware is OK.