On Fri, Oct 11, 2013 at 04:29:39PM -0400, Mark Lord wrote:

quoted

static int xx_alloc_msix_irqs(struct xx_dev *dev, int nvec)
{
	nvec = roundup_pow_of_two(nvec);	/* assume 0 > nvec <= 16 */

	xx_disable_all_irqs(dev);

	pci_lock_msi(dev->pdev);

	rc = pci_get_msix_limit(dev->pdev, nvec);
	if (rc < 0)
		goto err;

	nvec = min(nvec, rc);		/* if limit is more than requested */
	nvec = rounddown_pow_of_two(nvec);	/* (a) */

	xx_prep_for_msix_vectors(dev, nvec);

	rc = pci_enable_msix(dev->pdev, dev->irqs, nvec);	/* (b)	*/
	if (rc < 0)
		goto err;

	pci_unlock_msi(dev->pdev);

	dev->num_vectors = nvec;		/* (b) */
	return 0;

err:
	pci_unlock_msi(dev->pdev);

        kerr(dev->name, "pci_enable_msix() failed, err=%d", rc);
        dev->num_vectors = 0;
        return rc;
}

That would still need a loop, to handle the natural race between
the calls to pci_get_msix_limit() and pci_enable_msix() -- the driver and device
can and should fall back to a smaller number of vectors when pci_enable_msix() fails.

Could you please explain why the value returned by pci_get_msix_limit()
might change before pci_enable_msix() returned, while both protected by
pci_lock_msi()?

Anyway, although the loop-free code (IMHO) reads better, pci_lock_msi()
it is not a part of the original proposal and the more I think about it
the less I like it.

-- 
Regards,
Alexander Gordeev
agordeev@redhat.com