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

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:


static int xx_alloc_msix_irqs (struct xx_dev *dev, int nvec)
{
        xx_disable_all_irqs(dev);
        do {
                if (nvec < 2)
                        xx_prep_for_1_msix_vector(dev);
                else if (nvec < 4)
                        xx_prep_for_2_msix_vectors(dev);
                else if (nvec < 8)
                        xx_prep_for_4_msix_vectors(dev);
                else if (nvec < 16)
                        xx_prep_for_8_msix_vectors(dev);
                else
                        xx_prep_for_16_msix_vectors(dev);
                nvec = pci_enable_msix(dev->pdev, dev->irqs, dev->num_vectors);
        } while (nvec > 0);

        if (nvec) {
                kerr(dev->name, "pci_enable_msix() failed, err=%d", nvec);
                dev->num_vectors = 0;
                return nvec;
        }
        return 0;       /* success */
}

Yeah, that is a very good example.

I would move all xx_prep_for_<pow2>_msix_vector() functions to a single
helper i.e. xx_prep_msix_vectors(dev, ndev).

Considering also (a) we do not want to waste unused platform resources
associated with MSI-Xs and pull more quota than needed and (b) fixing
couple of bugs, this function could look like this:

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

	xx_disable_all_irqs(dev);

retry:
       	xx_prep_for_msix_vectors(dev, nvec);

	rc = pci_enable_msix(dev->pdev, dev->irqs, nvec);	/* (b) */
	if (rc > 0) {
		nvec = rounddown_pow_of_two(nvec);		/* (a) */
		goto retry;
	}

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

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

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;
}

-- 
Regards,
Alexander Gordeev
agordeev@redhat.com