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

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Date:	Wed, 18 Dec 2013 11:58:47 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <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" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v4 9/9] PCI/MSI: Introduce pci_auto_enable_msi*() family helpers

On Wed, Dec 18, 2013 at 6:23 AM, Alexander Gordeev <agordeev@...hat.com> wrote:
> 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.

I think this can still be done even with pci_enable_msix_range().
Here's what I'm thinking, tell me where I'm going wrong:

    rc = pci_enable_msix_range(dev->pdev, dev->irqs, 1, 16);
    if (rc < 0) { /* error */ }
    else { /* rc interrupts allocated */ }

If rc == 13 and the device can only use 8, the extra 5 would be
ignored and wasted.

If the waste is unacceptable, the driver can try this:

    rc = pci_enable_msix_range(dev->pdev, dev->irqs, 16, 16);
    if (rc < 0) {
        rc = pci_enable_msix_range(dev->pdev, dev->irqs, 8, 8);
        if (rc < 0) {
            rc = pci_enable_msix_range(dev->pdev, dev->irqs, 4, 4);
            ...
    }

    if (rc < 0) { /* error, couldn't allocate *any* interrupts */
    else { /* rc interrupts allocated (1, 2, 4, 8, or 16) */ }

Bjorn
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