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

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Date:	Fri, 20 Dec 2013 10:04:13 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Bjorn Helgaas <bhelgaas@...gle.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 Thu, Dec 19, 2013 at 02:37:22PM -0700, Bjorn Helgaas wrote:
> On Thu, Dec 19, 2013 at 6:42 AM, Alexander Gordeev <agordeev@...hat.com> wrote:
> > On Wed, Dec 18, 2013 at 11:58:47AM -0700, Bjorn Helgaas wrote:
> >> 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);
> >>             ...
> >>     }
> >
> > I have troubles with this fallback logic. On each failed step we get an
> > error and we do not know if this is indeed an error or an indication of
> > insufficient MSI resources. Even -ENOSPC would not tell much, since it
> > could be thrown from a lower level.
> >
> > By contrast, with the tri-state return value we can distinguish and bail
> > out on errors right away.
> 
> I thought the main point of this was to get rid of interfaces that
> were prone to misuse, and tri-state return values was a big part of
> that.  All we really care about in the driver is success/failure.  I'm
> not sure there's much to be gained by analyzing *why* we failed, and I
> think it tends to make uncommon error paths more complicated than
> necessary.  If we fail four times instead of bailing out after the
> first failure, well, that doesn't sound terrible to me.  The last
> failure can log the errno, which is enough for debugging.

Sure, the main point is to get rid of try-state interfaces. I just afraid
to throw out the baby with the bath water for unusual devices (which we do
not have in the tree).

I can only identify two downsides of the approach above - (a) repeated error
logging in a platform code (i.e. caused by -ENOMEM) and (b) repeated attempts
to enable MSI when the platform already reported a fatal error.

I think if a device needs an extra magic to enable MSI (i.e. writing to
specific registers etc.) it would be manageable with pci_enable_msix_range(),
but may be I am missing something?

So my thought is may be we deprecate the tri-state interfaces, but do not
do it immediately.

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