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SubjectRe: Interrupt sharing
Mahadev K Cholachagudda wrote:
>
[...]
> > When an interrupt is delivered, the kernel calls ALL interrupt handlers
> > registered for that interrupt. That means all drivers capable of
> > sharing interrupts should, ideally, have code in their interrupt handler
> > to exit ASAP if no work is necessary.
> >
> > status = RTL_R16(IntrStatus);
> > /* exit ASAP if no interrupt conditions (0), or
> > * if the hardware was unplugged (0xFFFF)
> > */
> > if ((status == 0) || (status == 0xFFFF))
> > return;
> >
>
> If this is the case, if i am not wrong the data which is to be delivered to
> the ISR will be emulated to the each drivers ISR. Is it true ?
>
No, there is no emulation because there is nothing to emulate.
There is no data "delivered" to an interrupt handler.

The interrupt handler checks to see if the _device_ it serves have
data, and handles that if necessary. Then the next handler for that
interrupt is called, and it will check its device and so on.

Note that you will never ever register two handlers for the same
interrupt from the _same_ device. That makes no sense. Interrupt
sharing happens when two different devices need to use the same
interrupt number, possibly because you have more devices
than available interrupts. (I.e. PCI have only four
interrupt lines, but you may have 6 pci slots in your
machine. Some will have to share.)

An example:
Machine have only one free interrupt line. A driver
for a disk controller loads, and register a handler
for that interrupt. Then a driver for a ethernet controller
is loaded, and register a handler for the same interrupt.


An interrupt happens. The kernel knows it belongs to one or
both of the two devices, but it can't know which one.
So it calls the handler for the first one, the disk controller.
That handler will check if something happened with the disk,
perhaps a completed write. In that case it does whatever
it needs, perhaps queuing another write. If nothing needs to be
done (because the disk didn't actually generate an interrupt) it
returns immediately.

When the handler returns
the kernel also calls the other handler for that interrupt, the
ethernet controller. It checks to see if anything happened
with the ethernet, perhaps we received a frame. The frame is
handled in that case. Or the routine just returns
if there was nothing to do.

As you see, no "emulation" of any kind, because each handler
work solely with its own device.

Helge Hafting
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