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

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Message-ID: <20131008043330.GF31666@concordia>
Date:	Tue, 8 Oct 2013 15:33:30 +1100
From:	Michael Ellerman <michael@...erman.id.au>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	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@...p-26-207.brq.redhat.com, "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 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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