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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: Michael Ellerman <michael@ellerman.id.au>
Date: Tue, 8 Oct 2013 15:33:30 +1100
Cc: linux-kernel@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>, Ralf Baechle <ralf@linux-mips.org>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, 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@dhcp-26-207.brq.redhat.com, "Inc." <pv-drivers@vmware.com>, linux-scsi@vger.kernel.org
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On Wed, Oct 02, 2013 at 12:29:04PM +0200, Alexander Gordeev wrote:
> This series is against "next" branch in Bjorn's repo:
> git://git.kernel.org/pub/scm/linux/kernel/git/helgaas/pci.git
> 
> Currently pci_enable_msi_block() and pci_enable_msix() interfaces
> return a error code in case of failure, 0 in case of success and a
> positive value which indicates the number of MSI-X/MSI interrupts
> that could have been allocated. The latter value should be passed
> to a repeated call to the interfaces until a failure or success:
> 
> 
>       for (i = 0; i < FOO_DRIVER_MAXIMUM_NVEC; i++)
>               adapter->msix_entries[i].entry = i;
> 
>       while (nvec >= FOO_DRIVER_MINIMUM_NVEC) {
>               rc = pci_enable_msix(adapter->pdev,
>                                    adapter->msix_entries, nvec);
>               if (rc > 0)
>                       nvec = rc;
>               else
>                       return rc;
>       }
> 
>       return -ENOSPC;
> 
> 
> This technique proved to be confusing and error-prone. Vast share
> of device drivers simply fail to follow the described guidelines.

To clarify "Vast share of device drivers":

 - 58 drivers call pci_enable_msix()
 - 24 try a single allocation and then fallback to MSI/LSI
 - 19 use the loop style allocation as above
 - 14 try an allocation, and if it fails retry once
 - 1  incorrectly continues when pci_enable_msix() returns > 0

So 33 drivers (> 50%) successfully make use of the "confusing and
error-prone" return value.

Another 24 happily ignore it, which is also entirely fine.

And yes, one is buggy, so obviously the interface is too complex. Thanks
drivers/ntb/ntb_hw.c

cheers

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