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

[ Upstream commit 2a1d728f20edeee7f26dc307ed9df4e0d23947ab ]

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: ff7693d ("ARM: meson: serial: add MesonX SoC on-chip uart driver")
Cc: [email protected]
Signed-off-by: Pavel Krasavin <[email protected]>
Reviewed-by: Neil Armstrong <[email protected]>
Reviewed-by: Dmitry Rokosov <[email protected]>

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

Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

drivers/tty/serial/meson_uart.c

@@ -371,10 +371,14 @@ static void meson_uart_set_termios(struct uart_port *port,
 	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);
 
@@ -670,6 +674,7 @@ static int meson_uart_probe(struct platform_device *pdev)
 	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");
@@ -697,6 +702,7 @@ static int meson_uart_probe(struct platform_device *pdev)
 		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]) {
 		dev_err(&pdev->dev, "port %d already allocated\n", pdev->id);
@@ -721,6 +727,8 @@ static int meson_uart_probe(struct platform_device *pdev)
 	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->dev = &pdev->dev;
 	port->line = pdev->id;
 	port->type = PORT_MESON;