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

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Date:	Wed, 2 Oct 2013 12:43:24 +1000
From:	Michael Ellerman <michael@...erman.id.au>
To:	Alexander Gordeev <agordeev@...hat.com>
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 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.

> 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.

> (Of course, we could allocate-deallocate MSIs at check time, but I think it
> is an overkill).

It's not only overkill, it's messing with the device behind the drivers
back, which is definitely a no-no in my opinion.
 
> > You'll also need to add another arch hook, for the quota check, and
> > we'll have to add it to our per-platform indirection as well.
> 
> Already, in a branch, hidden from Bjorn & Tejun eyes ;)
> 
> > All a lot of bother for no real gain IMHO.
> 
> Well, I do not have a strong opinion here. I leave it to the ones who have :)
> But few drivers have became clearer as result of this change (and messy ones
> are still messy).

Amen.

cheers
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