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SubjectRe: [PATCH-next 3/3] serial/sysrq: Add MAGIC_SYSRQ_SERIAL_SEQUENCE
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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

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