Re: [PATCH LINUX v4 06/13] tty: xuartps: Move request_irq to after setting up the HW

[Peter Hurley <peter@hurleysoftware.com>]

On 12/15/2015 07:41 AM, Sören Brinkmann wrote:
> On Thu, 2015-12-10 at 01:41PM -0800, Peter Hurley wrote:
>> On 12/05/2015 08:39 PM, Soren Brinkmann wrote:
>>> Request_irq() should be _after_ h/w programming, otherwise an
>>> interrupt could be triggered and in-progress before the h/w has been
>>> setup.
>>
>> Slight misunderstanding. My fault; I should have been more explicit.
>>
>> 1. Any setup necessary for the isr not to be confused and misdirect spurious
>>    interrupts (or hang) should be before installing the isr with request_irq()
>>    None of this code should trigger an interrupt.
>> 2. Clear pending interrupts
>> 3. Install the isr with request_irq()
>> 4. Enable interrupts
> 
> Isn't that what the startup function is doing now - more or less. I
> think 3 and 4 are swapped to release the lock and then do the
> request_irq, but I believe that should be OK.
> The startup function configures the HW. Clears the ISR. Enables the
> intended IRQs and then does the request_irq call.

If the driver enables interrupts before installing the isr with request_irq()
and an interrupt occurs there will the no handler to catch it and EOI the
device.


>> For extra safety, first disable interrupts before starting h/w programming.
> 
> It's done within spin_lock_irqsave, which gives us at least locally
> disabled IRQs. I guess we could add a disabling all IRQs in the UART
> core, but it should not really be necessary.

Similar issue.

What I mean is to mask interrupts from this device so that h/w programming
doesn't accidentally trigger an interrupt for which no isr is installed.

It's a bit overkill; that's why I said "extra safety".

Regards,
Peter Hurley


>> I would do the v5 series in the same order as the v3 series only up to
>> what I reviewed. Then do another series with the remainder plus new changes, ok?
> 
> Sure.
> 
> 	Sören
> 
>>
>> Regards,
>> Peter Hurley
>>
>>> Reported-by: Peter Hurley <peter@hurleysoftware.com>
>>> Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
>>> ---
>>> v4:
>>>  - this patch has been added. Thanks to Peter for pointing it out and providing
>>>    commit message
>>> ---
>>>  drivers/tty/serial/xilinx_uartps.c | 9 ++-------
>>>  1 file changed, 2 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/tty/serial/xilinx_uartps.c b/drivers/tty/serial/xilinx_uartps.c
>>> index 6ffd3bbe3e18..1e9053656610 100644
>>> --- a/drivers/tty/serial/xilinx_uartps.c
>>> +++ b/drivers/tty/serial/xilinx_uartps.c
>>> @@ -759,12 +759,7 @@ static void cdns_uart_set_termios(struct uart_port *port,
>>>  static int cdns_uart_startup(struct uart_port *port)
>>>  {
>>>  	unsigned long flags;
>>> -	unsigned int retval = 0, status = 0;
>>> -
>>> -	retval = request_irq(port->irq, cdns_uart_isr, 0, CDNS_UART_NAME,
>>> -								(void *)port);
>>> -	if (retval)
>>> -		return retval;
>>> +	unsigned int status = 0;
>>>  
>>>  	spin_lock_irqsave(&port->lock, flags);
>>>  
>>> @@ -818,7 +813,7 @@ static int cdns_uart_startup(struct uart_port *port)
>>>  
>>>  	spin_unlock_irqrestore(&port->lock, flags);
>>>  
>>> -	return retval;
>>> +	return request_irq(port->irq, cdns_uart_isr, 0, CDNS_UART_NAME, port);
>>>  }
>>>  
>>>  /**
>>>
>>