[RFC] nvme-pci does not disable interrupt-source in threaded mode

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

From drivers/nvme/host/pci.c:
| static int queue_request_irq(struct nvme_queue *nvmeq)
| {
|         struct pci_dev *pdev = to_pci_dev(nvmeq->dev->dev);
|         int nr = nvmeq->dev->ctrl.instance;
| 
|         if (use_threaded_interrupts) {
|                 return pci_request_irq(pdev, nvmeq->cq_vector, nvme_irq_check,
|                                 nvme_irq, nvmeq, "nvme%dq%d", nr, nvmeq->qid);

nvme_irq_check() checks if an interrupt is pending and does not disable
interrupt source if the interrupt is pending. It simply wakes the thread.
This works because the MSI-IRQ-controller is handled as edge and it does
not re-trigger. Should the IRQ fallback to non-MSI it would become LEVEL
and will keep the IRQ-line asserted so the threaded handler will no be reached.

One of these should be done:
- remove "use_threaded_interrupts" and rely simply on the "threadirqs"
  kernel command line.
- Let nvme_irq_check() disable the IRQ-source and nvme_irq() should
  enable it again once done.
- Pass IRQF_ONESHOT as irqflags.

Side note:
 msix_request_rcd_irq() (drivers/infiniband/hw/hfi1/msix.c) provides also
 both handlers and (according to the comment) receive_context_interrupt()
 disables the source before waking the thread. This one is fine and I
 did not find more users of pci_request_irq() providing both handlers.

Sebastian

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