Enable RS485 mode for max310x SPI-UART converter

Question

In my application I need RS485 interfaces. I am using some UARTs from am3352 but I need few more, so I'm trying to expand using SPI and max3109 chip.

I have successfully added max3109 to my device tree using module max310x - it shows two devices: /dev/ttyMAX0 and /dev/ttyMAX1. Here is the device tree fragment:

&spi1 {
    status = "okay";
    pinctrl-names = "default";
    pinctrl-0 = <&spi1_pins>;
    num_cs = <1>;
    cs-gpios = <&gpio2 17 0>;
    ti,pindir-d0-out-d1-in;

    max310x_0: max0@0 {
        compatible = "maxim,max3109";
        reg = <0>;
        spi-max-frequency = <24000000>;
        clocks = <&clk1m8>;
        clock-names = "xtal";
        interrupt-parent = <&gpio2>;
        interrupts = <12 IRQ_TYPE_EDGE_FALLING>;
        gpio-controller;
        #gpio-cells = <2>;
        clk1m8: clk1m8 {
            compatible = "fixed-clock";
            #clock-cells = <0>;
            clock-frequency = <1843200>;
        };
    };
};

and the pins:

spi1_pins: pinmux_spi1_pins {
    pinctrl-single,pins = <
        0x108 (PIN_INPUT_PULLUP | MUX_MODE2) /* (H16) gmii1_col.spi1_sclk */
        0x10c (PIN_INPUT_PULLUP | MUX_MODE2) /* (H17) gmii1_crs.spi1_d0 */
        0x110 (PIN_INPUT_PULLUP | MUX_MODE2) /* (J15) gmii1_rxer.spi1_d1 */
    >;
};

UARTs from max3109 are connected to rs232/rs485 converter with max3109's RTSn pins conected to both DE and RE pins.

The problem: UARTS on max3109 seems to work fine - both rs485 are transmitting data, but not reciving. Problem is that RTS is always at 0V level...

In UARTs from am3352 I am using in device tree the following property: "linux,rs485-enabled-at-boot-time". But adding it to main max310x_0 node is not giving any effect - this node is the expander node (containing 2 UARTs and gpio-controller), not the UART itself.

My idea is that I need to add a child-nodes for each UART and in it place the property "linux,rs485-enabled-at-boot-time". But I don't have a clue how to do it. I tried something like this:

&spi1 {
    status = "okay";
    pinctrl-names = "default";
    pinctrl-0 = <&spi1_pins>;
    num_cs = <1>;
    cs-gpios = <&gpio2 17 0>;
    ti,pindir-d0-out-d1-in;

    max310x_0: max0@0 {
        compatible = "maxim,max3109";
        reg = <0>;
        spi-max-frequency = <24000000>;
        clocks = <&clk1m8>;
        clock-names = "xtal";
        interrupt-parent = <&gpio2>;
        interrupts = <12 IRQ_TYPE_EDGE_FALLING>;
        gpio-controller;
        #gpio-cells = <2>;
        clk1m8: clk1m8 {
            compatible = "fixed-clock";
            #clock-cells = <0>;
            clock-frequency = <1843200>;
        };
        ttyMAX0 {
            linux,rs485-enabled-at-boot-time;
        };
        ttyMAX1 {
            linux,rs485-enabled-at-boot-time;
        };
    };
};

but it didn't worked.

My question: How am I supposed to add those child-nodes (if that's the proper way) and what should I place in them to make RTS work?

EDIT: after sawdust suggestion it seems it's impossible to add rs485 mode in device tree. So I tried to add this functionality to device tree and I think I'm starting to understand how things work down in here. To start with something I'm printing port.flags value to dmesg and it seems my little insertion works (a bit) - it changes the value depending on presence of linux,rs485-enabled-at-boot-time parameter in device tree. Here is the code I have inserted:

    if (of_property_read_bool(dev->of_node, "linux,rs485-enabled-at-boot-time")) 
    s->p[i].port.flags |= SER_RS485_ENABLED; 
    printk("s->p[i].port.flags is: %d\n",s->p[i].port.flags);

The value of port.flags toggles from 134225920 to 134225921 depending on presence of linux,rs485-enabled-at-boot-time. but the RTS pin still have constant 0V on my oscilloscope... I'm trying to figure out if SER_RS485_RTS_ON_SEND and SER_RS485_RTS_AFTER_SEND have something to do with this, but I'm prete sure it's only for reverting RTS signal.

*"How am I supposed to add those child-nodes (if that's the proper way)"* -- No, what you show is not correct. `linux,rs485-enabled-at-boot-time` is a property, and should be specified in the device node at the same level as `compatible = "maxim,max3109";` and `gpio-controller;`. However **max310x.c** does not call **uart_get_rs485_mode()** which would retrieve this property. You may have to use the runtime ioctl. AFAICT all ports of the device are configured identically by the DT. If you have doubts, then study the device driver. — sawdust, Dec 19 '19 at 09:23
I was just browsing the **max310x.c** looking for this. So ioctl seems to be only option... or maybe I'll try to patch it to add this call. — mslo, Dec 19 '19 at 11:26
You neglect to mention which kernel version you are using, but recent versions have put common rs485 code in **serial_core.c**. So unless you are using a older version that does not have this consolidation, then your edits are inappropriate. If I was working on this, I would write a program to verify that rs485 mode can be activated by the old-fashioned runtime ioctl. If necessary I'd put some **printk()** statements for debugging in the driver (dumping hex, not decimal, values). If xmit and receive function as expected, then work on the Device Tree. — sawdust, Dec 30 '19 at 08:34
You were right - ioctl is the simplest, and by that - the best solution. I'll write it in the answer with some code example I found useful for it, in case someone in future will have similar problem. Many thanks sawdust for you assistance. — mslo, Jan 10 '20 at 10:59

score 1 · Accepted Answer · answered Jan 17 '20 at 14:23

After few attempts IOCTL was the best and easiest solution. Here is some example code that helped me much. https://gist.github.com/amarburg/07564916d8d32e20e6ae375c1c83a995

It's basic example how to turn RS485 mode on and off using IOCTL and read it's current mode. Works with both CPUs internal UARTS and MAX3109.

Enable RS485 mode for max310x SPI-UART converter

1 Answers1