Rajendra Stalekar wrote:
Hi,
Wanted to know a scenario wherein an interrupt is detected by the
interrupt handler whose device is not the originator. How is that
possible ? We have a dev_id parameter to detect the device for any
interrupt.
This would be the case with shared interrupts. Note that every shared
interrupt handler will be invoked for each pending interrupt that occurs
on the line until we return IRQ_HANDLED from one of the interrupt
routines (and maybe clear the interrupt status in the device register -
se below). See LDD3 chapter 10 section 3 for more detail.
When we write our own interrupt handler , let’s say the interrupt line
is shared for that interrupt, yet we have the dev_id parameter for the
interrupt, so how can the interrupt handler detect the interrupt not
originating from the device. It will detect the interrupt only for the
device associated with dev_id.
See LDD3 chapter 10 section 5. In general for shared interrupts you
would need some kind of external indicator to know exactly which device
interrupted. That will be eg. a status register in either the interrupt
controller or device itself. Many times devices can report pending
interrupts via single status bit that can be read from a register inside
the device.
After establishing (inside the interrupt handler) that we were indeed
interrupted by the 'correct' device (eg. the interrupt bit is set) we
handle the interrupt, otherwise ignore the interrupt and possibly let
other interrupt handlers registered for the same IRQ number do the job.
It is worth mentioning that some devices may need some kind of procedure
to clear the interrupt in the device itself (setting/clearing the
interrupt status bit) otherwise interrupt would be fired indefinitely.
When registering the interrupt handler some kind of private (and unique)
data for dev_id *must* be specified. As LDD3 suggests the best candidate
for this would be pointer to device data structure. To finally answer
the question: I don't think it is possible for interrupt handler to work
this out for himself if not assisted from the device itself - eg. by
reading the device status register or similar.
LDD3 is little vague on this subject, it says that free_irq is the main
reason for use of unique dev_id, while each interrupt handler is on its
own when recognizing the device which fired the interrupt. Since every
interrupt handler receives its own dev_id when invoked, its impossible
to know what other devices might be also hooked to the same IRQ line.
regards,
hinko
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