linux-kernel - Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi

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Message-ID: <20131002071000.GA29860@dhcp-26-207.brq.redhat.com>
Date:	Wed, 2 Oct 2013 09:10:00 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Michael Ellerman <michael@...erman.id.au>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Joerg Roedel <joro@...tes.org>,
	Jan Beulich <JBeulich@...e.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi_cap() interface

On Wed, Oct 02, 2013 at 12:43:24PM +1000, Michael Ellerman wrote:
> On Tue, Oct 01, 2013 at 12:35:27PM +0200, Alexander Gordeev wrote:
> > On Tue, Oct 01, 2013 at 05:51:33PM +1000, Michael Ellerman wrote:
> > > The disadvantage is that any restriction imposed on us above the quota
> > > can only be reported as an error from pci_enable_msix().
> > > 
> > > The quota code, called from pci_get_msix_limit(), can only do so much to
> > > interogate firmware about the limitations. The ultimate way to check if
> > > firmware will give us enough MSIs is to try and allocate them. But we
> > > can't do that from pci_get_msix_limit() because the driver is not asking
> > > us to enable MSIs, just query them.
> > 
> > If things are this way then pci_enable_msix() already exposed to this
> > problem internally on pSeries.
> > 
> > I see that even successful quota checks in rtas_msi_check_device() and
> > rtas_setup_msi_irqs() do not guarantee (as you say) that firmware will
> > give enough MSIs. Hence, pci_enable_msix() might fail even though the
> > its quota checks succeeded.
> 
> Yes, but it can report that failure to the caller, which can then retry.

If a driver wants to retry after a failure it is up to the driver (but why?).
The current guidlines state:

"If this function returns a negative number, it indicates an error and
the driver should not attempt to allocate any more MSI-X interrupts for
this device."

Anyway, what number could the driver retry with after it got a negative errno?

> > Therefore, nothing will really change if we make pci_get_msix_limit() check
> > quota and hope the follow-up call to pci_enable_msix() succeeded.
> 
> No that's not equivalent. Under your scheme if pci_enable_msix() fails
> then the caller just bails, it will never try again with a lower number.

Currently under the very same circumstances (the quota check within
rtas_setup_msi_irqs() returned Q vectors while the firmware has only F
vectors to allocate and Q > F) rtas_setup_msi_irqs() fails, pci_enable_msix()
fails, the caller bails and never try again with a lower number.

Am I missing something here?

> cheers

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