Re: [PATCH] synchronize_irq needs a barrier

I have read this thread and I concluded few things:

1) It is impossible to know that the card won't send more interrupts:
Even if I do a read from the device, the IRQ can be pending in the bus/APIC
It is even possible (and likely) that the IRQ line will be shared, thus the 
handler can be called by non-relevant device.

2) the synchronize_irq(); in .suspend is useless:
an IRQ can happen immediately after this synchronize_irq();
and interrupt even the .suspend()
(At least theoretically)

It's not totally useless not. It guarantees that by the time your
are out of your suspend(), a simultaneous instance of the IRQ handler
is either finished, or it (or any subsequent occurence) have seen
your insuspend flag set to 1.

Thus I now understand that .suspend() should do:

	saa_writel(SAA7134_IRQ1, 0);
	saa_writel(SAA7134_IRQ2, 0);
	saa_writel(SAA7134_MAIN_CTRL, 0);

	dev->insuspend = 1;
	smp_wmb();

	/* at that point the _request to disable card's IRQs was issued, we don't know 
	   that there will be no irqs anymore.
	   the smp_mb(); guaranties that the IRQ handler will bail out in that case. */
	
	/* .......*/

	pci_save_state(pci_dev);
	pci_set_power_state(pci_dev, pci_choose_state(pci_dev, state));
	return 0;

The above doesn't handle the case where the IRQ handle just passed the
if (insuspend) test. You may end up calling pci_set_power_state() while
in the middle of some further HW accesses in the handler.

You still need synchronize_irq() for that reason. Or use a spinlock.

and the interrupt handler:

	smp_rmb();
	if (dev->insuspend)
		goto out;

Am I right?

Not quite :-)

Also not that the work we are doing with synchronize_irq, if it gets
merged, renders it unnecessary for you to add those two memory barriers,
synchronize_irq will pretty much do the ordering for you.

Cheers,
Ben.