linux-kernel - Re: [PATCH-next 3/3] serial/sysrq: Add MAGIC_SYSRQ_SERIAL

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Date:   Fri, 10 Jan 2020 22:32:34 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Jiri Slaby <jslaby@...e.com>,
        Vasiliy Khoruzhick <vasilykh@...sta.com>,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH-next 3/3] serial/sysrq: Add MAGIC_SYSRQ_SERIAL_SEQUENCE

On 1/10/20 4:46 PM, Greg Kroah-Hartman wrote:
[..]
>> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
>> index 6ac9dfed3423..f70eba032d0b 100644
>> --- a/drivers/tty/serial/serial_core.c
>> +++ b/drivers/tty/serial/serial_core.c
>> @@ -3081,6 +3081,38 @@ void uart_insert_char(struct uart_port *port, unsigned int status,
>>  }
>>  EXPORT_SYMBOL_GPL(uart_insert_char);
>>  
>> +const char sysrq_toggle_seq[] = CONFIG_MAGIC_SYSRQ_SERIAL_SEQUENCE;
>> +
>> +static void uart_sysrq_on(struct work_struct *w)
>> +{
>> +	sysrq_toggle_support(1);
>> +	pr_info("SysRq is enabled by magic sequience on serial\n");
> 
> Do we want to say what serial port it is enabled on?

Makes sense, will add.

> And why is this done in a workqueue?

uart_try_toggle_sysrq() sometimes is called under
spin_lock_irqsave(&port->lock, flags);

And sysrq_toggle_support() calls input_register_handler() internally
which can sleep.

>> +}
>> +static DECLARE_WORK(sysrq_enable_work, uart_sysrq_on);
>> +
>> +static int uart_try_toggle_sysrq(struct uart_port *port, unsigned int ch)
>> +{
>> +	if (sysrq_toggle_seq[0] == '\0')
>> +		return 0;
> 
> Is constantly checking the data stream like this going to slow things
> down overall?  Ah, we are just checking this after BREAK, right?  So
> that hopefully will not be that bad...

Yes, it's after BREAK. In my POV it's fine as originally it would cause
sysrq handler being called (if sysrq is enabled).

> 
>> +
>> +	BUILD_BUG_ON(ARRAY_SIZE(sysrq_toggle_seq) >= sizeof(port->sysrq_seq)*U8_MAX);
>> +	if (sysrq_toggle_seq[port->sysrq_seq] != ch) {
>> +		port->sysrq_seq = 0;
>> +		return 0;
>> +	}
>> +
>> +	/* Without the last \0 */
>> +	if (++port->sysrq_seq < (ARRAY_SIZE(sysrq_toggle_seq) - 1)) {
>> +		port->sysrq = jiffies + HZ*5;
> 
> 5 second delay?  You should document what this value is for somewhere
> here...

Fair enough, I'll add
#define SYSRQ_TIMEOUT	(HZ*5)

And use it in uart_handle_break() too.

>> @@ -3090,9 +3122,13 @@ int uart_handle_sysrq_char(struct uart_port *port, unsigned int ch)
>>  		return 0;
>>  
>>  	if (ch && time_before(jiffies, port->sysrq)) {
>> -		handle_sysrq(ch);
>> -		port->sysrq = 0;
>> -		return 1;
>> +		if (sysrq_get_mask()) {
>> +			handle_sysrq(ch);
>> +			port->sysrq = 0;
>> +			return 1;
>> +		}
> 
> Isn't this change to test for sysrq_get_mask() a different change than
> checking for the "magic" data stream?

It's for the case when sysrq is already enabled.
Than sysrq_get_mask() will return something and it makes uart call
handle_sysrq() instead of checking the toggle sequence.

>> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
>> index 255e86a474e9..1f4443db5474 100644
>> --- a/include/linux/serial_core.h
>> +++ b/include/linux/serial_core.h
>> @@ -243,10 +243,10 @@ struct uart_port {
>>  	unsigned long		sysrq;			/* sysrq timeout */
>>  	unsigned int		sysrq_ch;		/* char for sysrq */
>>  	unsigned char		has_sysrq;
>> +	unsigned char		sysrq_seq;		/* index in sysrq_toggle_seq */
>>  
>>  	unsigned char		hub6;			/* this should be in the 8250 driver */
>>  	unsigned char		suspended;
>> -	unsigned char		unused;
> 
> This is an unrelated change, let's leave it for a different patch that
> cleans up the layout of this structure, ok?

Yes, sure.

Thanks,
          Dmitry