On Thu, Feb 10, 2005 at 02:08:16PM -0800, Christopher Hoover wrote:
> > Greg:
> >
> > This patch contains the changes to the ohci-sa1111 driver.
> >
> > Alan Stern
>
>
> This patch introduces several paths in the interrupt handler that return
> IRQ_NONE. In the original code, we took care (in usb_hcd_sa1111_hcim_irq)
> to always return IRQ_HANDLED, no matter what. I don't recall offhand why we
> did this, but I'm mildly hesitant about this part of the patch given that
> the SA-1111 is a quirky beast, especially re: interrupts.
It's because edge triggered interrupts are fundamentally unreliable with
"bad interrupt" detection.
Consider:
while (interrupt pending) {
clear it;
service it;
}
and consider what happens in hardware when an interrupt occurs after
clearing the original source. The new interrupt triggers the interrupt
circuitry, and, because this interrupt is being serviced, we remember
that it happened. Meanwhile, we loop and find this new interrupt
pending and service it. However, the generic IRQ code has no way to
clear this condition.
Once the interrupt has completed, we leave the driver handler. The
generic IRQ code notices we have a pending interrupt, and re-calls
the handler. The handler finds no work to do, and would normally
return IRQ_NONE. At that point, we scream about a bad interrupt.
One thing someone might suggest is why we remember such interrupts.
Consider the alternative case where the interrupt occurs immediately
after we've determined that there is no interrupt pending in the
driver specific handler, but before we've returned to the generic
IRQ code. Not remembering the pending interrupt means we drop
pending interrupts, possibly causing that device to stop responding.
So, essentially, the whole irqreturn_t/IRQ_HANDLED/IRQ_NONE doesn't
suit edge-triggered interrupt sources on ARM. I've been debating
for some time to make the return value irrelevant with such
triggered sources - not doing so currently means people end up
passing noirqdebug to the kernel just to get a stable system. This
is obviously not the point of the original idea behind this.
--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 PCMCIA - http://pcmcia.arm.linux.org.uk/
2.6 Serial core
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