linux-kernel - Re: [PATCH v4 9/9] PCI/MSI: Introduce pci_auto_enable

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Message-ID: <20131218132349.GA29552@dhcp-26-207.brq.redhat.com>
Date:	Wed, 18 Dec 2013 14:23:49 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	linux-kernel@...r.kernel.org,
	Michael Ellerman <michael@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Tejun Heo <tj@...nel.org>,
	Ben Hutchings <bhutchings@...arflare.com>,
	David Laight <David.Laight@...LAB.COM>,
	Mark Lord <kernel@...rt.ca>, "H. Peter Anvin" <hpa@...or.com>,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH v4 9/9] PCI/MSI: Introduce pci_auto_enable_msi*() family
 helpers

On Tue, Dec 17, 2013 at 05:30:02PM -0700, Bjorn Helgaas wrote:

Hi Bjorn,

Thank you for the review!

Sorry for a heavy skipping - I just wanted to focus on a principal
moment in your suggestion and then go on with the original note.

> I only see five users of pci_enable_msi_block() (nvme, ath10k, wil6210,
> ipr, vfio); we can easily convert those to use pci_enable_msi_range() and
> then remove pci_enable_msi_block().

> It would be good if pci_enable_msix() could be implemented in terms of
> pci_enable_msix_range(nvec, nvec), with a little extra glue to handle the
> positive return values.

So you want to get rid of the tri-state "low-level" pci_enable_msi_block()
and pci_enable_msix(), right? I believe we can not do this, since we need
to support a non-standard hardware which (a) can not be asked any arbitrary
number of vectors within a range and (b) needs extra magic to enable MSI
operation.

I.e. below is a snippet from a real device driver Mark Lord has sent in a
previous conversation:

        xx_disable_all_irqs(dev);
        do {
            	if (nvec < 2)
                        xx_prep_for_1_msix_vector(dev);
                else if (nvec < 4)
                        xx_prep_for_2_msix_vectors(dev);
                else if (nvec < 8)
                        xx_prep_for_4_msix_vectors(dev);
                else if (nvec < 16)
                        xx_prep_for_8_msix_vectors(dev);
                else
                    	xx_prep_for_16_msix_vectors(dev);
                nvec = pci_enable_msix(dev->pdev, dev->irqs, dev->num_vectors);
       	} while (nvec > 0);

The same probably could have been done with pci_enable_msix_range(nvec, nvec)
call and checking for -ENOSPC errno, but IMO it would be less graceful and
reliable, since -ENOSPC might come from anywhere.

IOW, I believe we need to keep the door open for custom MSI-enablement (loop)
implementations.

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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