On 13-10-11 04:41 AM, Alexander Gordeev wrote:

On Thu, Oct 10, 2013 at 07:17:18PM -0400, Mark Lord wrote:

quoted

Just to help us all understand "the loop" issue..

Here's an example of driver code which uses the existing MSI-X interfaces,
for a device which can work with either 16, 8, 4, 2, or 1 MSI-X interrupt.
This is from a new driver I'm working on right now:

..

Now, this is a loop-free alternative:

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.