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

To: Alexander Gordeev <agordeev@redhat.com>
Subject: Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement pattern
From: Mark Lord <kernel@start.ca>
Date: Fri, 11 Oct 2013 16:29:39 -0400
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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On 13-10-11 04:41 AM, Alexander Gordeev wrote:
> 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:
..
> 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;
> }

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.

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