[PATCH v4] tty: serial: meson: fix hard LOCKUP on crtscts mode

[Neil Armstrong]

On 13/10/2023 16:18, Dmitry Rokosov wrote:
> I believe it would be necessary to include a 'Fixes' tag for that. Neil,
> what do you think?
> Since a HARDLOCKUP is an undesirable situation, I don't think we want to
> have it in the LTS kernels either.

Yes, and please keep the previous Reviewed-by Dmitry and I added on v3 and send a v5 with the Fixes added.

It should be:
Fixes: ff7693d079e5 ("ARM: meson: serial: add MesonX SoC on-chip uart driver")

Hopefully it will be the last iteration.

Thanks,
Neil
> 
> On Fri, Oct 13, 2023 at 11:48:39AM +0000, Pavel Krasavin wrote:
>> From: Pavel Krasavin <pkrasavin at imaqliq.com>
>>
>> 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>
>> ---
>> v4: More correct patch subject according to Jiri's note
>> v3: https://lore.kernel.org/lkml/OF6CF5FFA0.CCFD0E8E-ON00258A46.00549EDF-00258A46.0054BB62@gdc.ru/
>> "From:" line added to the mail
>> v2: https://lore.kernel.org/lkml/OF950BEF72.7F425944-ON00258A46.00488A76-00258A46.00497D44@gdc.ru/
>> braces for single statement removed according to Dmitry's note
>> v1: https://lore.kernel.org/lkml/OF28B2B8C9.5BC0CD28-ON00258A46.0037688F-00258A46.0039155B@gdc.ru/
>> ---
>>
>> --- a/drivers/tty/serial/meson_uart.c	2023-10-12 15:44:02.410538523 +0300
>> +++ b/drivers/tty/serial/meson_uart.c	2023-10-12 15:58:06.242395253 +0300
>> @@ -380,10 +380,14 @@ 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;
>> +	} else {
>>   		val |= AML_UART_TWO_WIRE_EN;
>> +	}
>>   
>>   	writel(val, port->membase + AML_UART_CONTROL);
>>   
>> @@ -705,6 +709,7 @@ 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 +737,7 @@ 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 +768,8 @@ 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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>