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
[email protected]
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