tty: serial: meson: fix hard LOCKUP on crtscts mode

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.

Fixes: ff7693d079e5 ("ARM: meson: serial: add MesonX SoC on-chip uart driver")
Cc: [e-mail]
Signed-off-by: Pavel Krasavin <[e-mail]>
Reviewed-by: Neil Armstrong <[e-mail]>
Reviewed-by: Dmitry Rokosov <[e-mail]>

v6: stable tag added
v5: https://lore.kernel.org/lkml/[e-mail]/
added missed Reviewed-by tags, Fixes tag added according to Dmitry and Neil notes
v4: https://lore.kernel.org/lkml/[e-mail]/
More correct patch subject according to Jiri's note
v3: https://lore.kernel.org/lkml/[e-mail]/
"From:" line added to the mail
v2: https://lore.kernel.org/lkml/[e-mail]/
braces for single statement removed according to Dmitry's note
v1: https://lore.kernel.org/lkml/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]

Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/meson_uart.c

···
 	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;
+	} else {
 		val |= AML_UART_TWO_WIRE_EN;
+	}
 
 	writel(val, port->membase + AML_UART_CONTROL);
 
···
 	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");
···
 		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,
···
 	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;