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

To: Mark Lord <kernel@start.ca>
Subject: Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement pattern
From: Alexander Gordeev <agordeev@redhat.com>
Date: Tue, 15 Oct 2013 17:30:29 +0200
Cc: "H. Peter Anvin" <hpa@zytor.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, linux-kernel@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>, Ralf Baechle <ralf@linux-mips.org>, Michael Ellerman <michael@ellerman.id.au>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>, Dan Williams <dan.j.williams@intel.com>, Andy King <acking@vmware.com>, Jon Mason <jon.mason@intel.com>, Matt Porter <mporter@kernel.crashing.org>, linux-pci@vger.kernel.org, linux-mips@linux-mips.org, linuxppc-dev@lists.ozlabs.org, linux390@de.ibm.com, linux-s390@vger.kernel.org, x86@kernel.org, linux-ide@vger.kernel.org, iss_storagedev@hp.com, linux-nvme@lists.infradead.org, linux-rdma@vger.kernel.org, netdev@vger.kernel.org, e1000-devel@lists.sourceforge.net, linux-driver@qlogic.com, Solarflare linux maintainers <linux-net-drivers@solarflare.com>, "VMware, Inc." <pv-drivers@vmware.com>, linux-scsi@vger.kernel.org
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References: <cover.1380703262.git.agordeev@redhat.com> <5254D397.9030307@zytor.com> <1381292648.645.259.camel@pasglop> <20131010101704.GC11874@dhcp-26-207.brq.redhat.com> <5256D5AB.4050105@zytor.com> <20131010180704.GA15719@dhcp-26-207.brq.redhat.com> <5257357E.8080506@start.ca> <20131011084108.GA25702@dhcp-26-207.brq.redhat.com> <52585FB3.7080508@start.ca>
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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

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