Re: /dev/tty01 will not open on Beaglebone Black

[RVP <rvp%SDF.ORG@localhost>]

On Thu, 31 Aug 2023, Brook Milligan wrote:

> I have attached the test program below, and would appreciate more eyes on it  [...]

That looks OK apart from a few minor issues:

- If you want to turn off non-blocking, use the standard idiom instead of
  fcntl(fd,F_SETFL,0):

	int flg = fcntl(fd, F_GETFL, 0);
	fcntl(fd, F_SETFL, flg & ~O_NONBLOCK);

- Call cfmakeraw() _before_ changing settings (not after) because it modifies
  some bits (eg. CS8, ~PARENB) itself (see: src/lib/libc/termios/cfmakeraw.c)

> However, a write fails and hangs the board hard: no keyboard response, no response to pings.

As the others have said, this looks like a kernel issue. Apropos which: from
your `am335x-boneblack-comm_cape.dts' file:

```
&am33xx_pinmux {
        bborg_comms_rs485_pins: pinmux_comms_rs485_pins {
                pinctrl-single,pins = <
                        0x074 (PIN_OUTPUT | MUX_MODE6)  /* P9_13: gpmc_wpn.uart4_txd_mux2 */
                        0x070 (PIN_INPUT  | MUX_MODE6)  /* P9_11: gpmc_wait0.uart4_rxd_mux2 */
                >;
        };
};
```

Is that `RS-485' there correct? Shouldn't it be UART or RS-232 or something?

Following the links for the Comms cape from beagleboard.org, I landed here:

https://elinux.org/Beagleboard:BeagleBone_cape_interface_spec#UART

where the `BONE-UART4.dts' has this:

```
&ocp {
    P9_13_pinmux { pinctrl-0 = <&P9_13_uart_pin>; };    /* UART TX*/
    P9_11_pinmux { pinctrl-0 = <&P9_11_uart_pin>; };    /* UART RX*/
};

&bone_uart_4 {
    status = "okay";
};
```

There is also the `BB-UART4-00A0.dts' file here (where you got your .dts from):

https://github.com/beagleboard/bb.org-overlays/blob/master/src/arm/BB-UART4-00A0.dts

with different contents:

```
/*
 * Free up the pins used by the cape from the pinmux helpers.
 */
&ocp {
	P9_13_pinmux { status = "disabled"; };	/* P9_13: uart4_txd */
	P9_11_pinmux { status = "disabled"; };	/* P9_11: uart4_rxd */
};

&am33xx_pinmux {
	bb_uart4_pins: pinmux_bb_uart4_pins {
		pinctrl-single,pins = <
			AM33XX_PADCONF(AM335X_PIN_GPMC_WPN, PIN_OUTPUT, MUX_MODE6)	/* P9_13 gpmc_wpn.uart4_txd_mux2 */
			AM33XX_PADCONF(AM335X_PIN_GPMC_WAIT0, PIN_INPUT, MUX_MODE6)	/* P9_13 gpmc_wait0.uart4_rxd_mux2 */
		>;
	};
};

&uart4 {
	/* sudo agetty 115200 ttyS4 */
	status = "okay";
	pinctrl-names = "default";
	pinctrl-0 = <&bb_uart4_pins>;
};
```

No idea which is right (not an ARM-guy!). The folks on port-arm@ would know.

HTH,

-RVP