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

From: [email-masked] (Frank Rowand)
Date: Fri, 31 Oct 2014 11:08:15 -0700

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.
 
> 
>>
>>>
>>>
>>>> +			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.
>>>
>>
> 
> 
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