Subject: [PATCH 2/2] tty: serial: msm: Support sysrq on uartDM devices

From: [email-masked] (Daniel Thompson)
Date: Fri, 31 Oct 2014 09:43:07 +0000

On 31/10/14 06:41, Stephen Boyd wrote:
> On 10/30, Daniel Thompson wrote:
>> On 29/10/14 18:14, Stephen Boyd wrote:
>>> +		r_count = min_t(int, count, sizeof(buf));
>>> +
>>> +		for (i = 0; i < r_count; i++) {
>>> +			char flag = TTY_NORMAL;
>>>  
>>> -		/* TODO: handle sysrq */
>>> -		tty_insert_flip_string(tport, buf, min(count, 4));
>>> -		count -= 4;
>>> +			if (msm_port->break_detected && buf[i] == 0) {
>>> +				port->icount.brk++;
>>> +				flag = TTY_BREAK;
>>> +				msm_port->break_detected = false;
>>> +				if (uart_handle_break(port))
>>> +					continue;
>>> +			}
>>> +
>>> +			if (!(port->read_status_mask & UART_SR_RX_BREAK))
>>> +				flag = TTY_NORMAL;
>>
>> flag is already known to be TTY_NORMAL.
> 
> Huh? If we detected a break we would set the flag to TTY_BREAK
> and if uart_handle_break() returned 0 (perhaps sysrq config is
> diasbled) then we would get down here, and then we want to reset
> the flag to TTY_NORMAL if the read_status_mask bits indicate that
> we want to skip checking for breaks. Otherwise we want to
> indicate to the tty layer that it's a break character.

Agreed. Sorry for noise.

It now reaches the level of silly quibble (meaning I won't bother to
raise the issue again if there is a v2 patch) but perhaps updating the
flag after the continue would be easier to read.

>>> +
>>> +			spin_unlock(&port->lock);
>>
>> Is it safe to unlock at this point? count may no longer be valid when we
>> return.
> 
> Can you explain further? If it actually isn't valid something
> needs to be done. I believe other serial drivers are doing this
> sort of thing though so it doesn't seem that uncommon (of course
> those drivers could also be broken I suppose).

Calling spin_unlock() means we are allow code to alter the state of the
UART. In particular the subsequent call to uart_handle_sysrq_char() can
make significant changes to the FIFO state (by calling the poll_char
functions). Given count is shadowing the FIFO state, when we retake the
lock I think it is possible for count to no longer be valid.

> 
>>
>>
>>> +			sysrq = uart_handle_sysrq_char(port, buf[i]);
>>> +			spin_lock(&port->lock);
>>> +			if (!sysrq)
>>> +				tty_insert_flip_char(tport, buf[i], flag);
>>
>> flag has a constant value here.
>>
>