linux-kernel - Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement pattern

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Date:	Tue, 15 Oct 2013 17:30:29 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Mark Lord <kernel@...rt.ca>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Michael Ellerman <michael@...erman.id.au>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Andy King <acking@...are.com>, Jon Mason <jon.mason@...el.com>,
	Matt Porter <mporter@...nel.crashing.org>,
	linux-pci@...r.kernel.org, linux-mips@...ux-mips.org,
	linuxppc-dev@...ts.ozlabs.org, linux390@...ibm.com,
	linux-s390@...r.kernel.org, x86@...nel.org,
	linux-ide@...r.kernel.org, iss_storagedev@...com,
	linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
	netdev@...r.kernel.org, e1000-devel@...ts.sourceforge.net,
	linux-driver@...gic.com,
	Solarflare linux maintainers <linux-net-drivers@...arflare.com>,
	"VMware, Inc." <pv-drivers@...are.com>, linux-scsi@...r.kernel.org
Subject: Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement
 pattern

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@...hat.com
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