[PATCH] tty: serial: meson: hard LOCKUP on crtscts mode

[Dmitry Rokosov]

Hello Pavel,

On Thu, Oct 12, 2023 at 10:23:30AM +0000, pkrasavin at imaqliq.ru wrote:
> There might be hard lockup if we set crtscts mode on port without RTS/CTS configured:
> 
> # stty -F /dev/ttyAML6 crtscts; echo 1 > /dev/ttyAML6; echo 2 > /dev/ttyAML6
> [   95.890386] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> [   95.890857] rcu:     3-...0: (201 ticks this GP) idle=e33c/1/0x4000000000000000 softirq=5844/5846 fqs=4984
> [   95.900212] rcu:     (detected by 2, t=21016 jiffies, g=7753, q=296 ncpus=4)
> [   95.906972] Task dump for CPU 3:
> [   95.910178] task:bash            state:R  running task     stack:0     pid:205   ppid:1      flags:0x00000202
> [   95.920059] Call trace:
> [   95.922485]  __switch_to+0xe4/0x168
> [   95.925951]  0xffffff8003477508
> [   95.974379] watchdog: Watchdog detected hard LOCKUP on cpu 3
> [   95.974424] Modules linked in: 88x2cs(O) rtc_meson_vrtc
> 
> Possible solution would be to not allow to setup crtscts on such port.
> 
> Tested on S905X3 based board.
> 
> Signed-off-by: Pavel Krasavin <pkrasavin at imaqliq.com>
> 
> --- a/drivers/tty/serial/meson_uart.c	2023-08-22 12:46:50.933814528 +0300
> +++ b/drivers/tty/serial/meson_uart.c	2023-08-22 14:48:15.593169948 +0300
> @@ -380,10 +380,15 @@ static void meson_uart_set_termios(struc
>  	else
>  		val |= AML_UART_STOP_BIT_1SB;
>  
> -	if (cflags & CRTSCTS)
> -		val &= ~AML_UART_TWO_WIRE_EN;
> -	else
> +	if (cflags & CRTSCTS) {
> +		if (port->flags & UPF_HARD_FLOW) {
> +			val &= ~AML_UART_TWO_WIRE_EN;
> +		} else {
> +			termios->c_cflag &= ~CRTSCTS;
> +		}

Please do not use braces where is single statement. In other words:

	if (cflags & CRTSCTS) {
		if (port->flags & UPF_HARD_FLOW)
			val &= ~AML_UART_TWO_WIRE_EN;
		else
			termios->c_cflag &= ~CRTSCTS;
	} else {
		val |= AML_UART_TWO_WIRE_EN;
	}

> +	} else {
>  		val |= AML_UART_TWO_WIRE_EN;
> +	}
>  
>  	writel(val, port->membase + AML_UART_CONTROL);
>  
> @@ -705,6 +710,7 @@ static int meson_uart_probe(struct platf
>  	u32 fifosize = 64; /* Default is 64, 128 for EE UART_0 */
>  	int ret = 0;
>  	int irq;
> +	bool has_rtscts;
>  
>  	if (pdev->dev.of_node)
>  		pdev->id = of_alias_get_id(pdev->dev.of_node, "serial");
> @@ -732,6 +738,7 @@ static int meson_uart_probe(struct platf
>  		return irq;
>  
>  	of_property_read_u32(pdev->dev.of_node, "fifo-size", &fifosize);
> +	has_rtscts = of_property_read_bool(pdev->dev.of_node, "uart-has-rtscts");
>  
>  	if (meson_ports[pdev->id]) {
>  		return dev_err_probe(&pdev->dev, -EBUSY,
> @@ -762,6 +769,8 @@ static int meson_uart_probe(struct platf
>  	port->mapsize = resource_size(res_mem);
>  	port->irq = irq;
>  	port->flags = UPF_BOOT_AUTOCONF | UPF_LOW_LATENCY;
> +	if (has_rtscts)
> +		port->flags |= UPF_HARD_FLOW;
>  	port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_MESON_CONSOLE);
>  	port->dev = &pdev->dev;
>  	port->line = pdev->id;
> 
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-- 
Thank you,
Dmitry