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

From: [email-masked] (Frank Rowand)
Date: Mon, 03 Nov 2014 19:00:52 -0800

On 11/3/2014 2:05 AM, Daniel Thompson wrote:
> On 31/10/14 18:08, Frank Rowand wrote:
>> On 10/31/2014 2:43 AM, Daniel Thompson wrote:
>>> 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.
>>
>> uart_handle_sysrq_char() will not _read_ from the serial port.  So it will
>> not directly modify the FIFO state.
> 
> poll_char does not read from the FIFO? Since when?
> 
> SysRq-g will enter cause the system to enter kdb/kgdb from within
> uart_handle_sysrq_char().

Aarrgh.  You are correct.  I overlooked the obvious SysRq-g.

/me searches for paper bag.

-Frank