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Date: Tue, 15 Oct 2013 17:30:29 +0200
From: Alexander Gordeev <agordeev@redhat.com>
To: Mark Lord <kernel@start.ca>
Subject: Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement
 pattern
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On Fri, Oct 11, 2013 at 04:29:39PM -0400, Mark Lord wrote:
> > 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