title: Should my interrupt handler disable interrupts or does the ARM processor do it automatically?
body: <p>Our group is using a custom driver to interface four MAX3107 UARTs on a shared I2C bus.  The interrupts of the four MAX3107's are connected (i.e. shared interrupt via logic or'ing)) to a GPIO pin on the ARM9 processor (LPC3180 module).  When one or more of these devices interrupt, they pull the GPIO line, which is configured as a level-sensitive interrupt, low.  My question concerns the need, or not, to disable the specific interrupt line in the handler code. (I should add that we are running Linux 2.6.10).</p>
  
  <p>Based on my reading of several ARM-specific app notes on interrupts, it seems that when the ARM processor receives an interrupt, it automatically disables (masks?) the corresponding interrupt line (in our case this would seem to be the line corresponding to the GPIO pin we selected).  If this is true, then it seems that we should not have to disable interrupts for this GPIO pin in our interrupt handler code as doing so would seem redundant (though it seems to work okay).  Stated differently, it seems to me that if the ARM processor automatically disables the GPIO interrupt upon an interrupt occurring, then if anything, our interrupt handler code should only have to re-enable the interrupt once the device is serviced.</p>
  
  <p>The interrupt handler code that we are using includes <code>disable_irq_nosync(irqno);</code> at the very beginning of the handler and a corresponding <code>enable_irq()</code> at the end of the handler.  If the ARM processor has already disabled the interrupt line (in hardware), what is the effect of these calls (i.e. a call to <code>disable_irq_nosync()</code> followed by a call to <code>enable(irq())</code>?</p>