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

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Date:	Wed, 18 Sep 2013 18:50:45 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Michael Ellerman <michael@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.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, Sep 18, 2013 at 09:22:31AM -0500, Tejun Heo wrote:
> > > We have a small number of MSIs available, limited by hardware &
> > > firmware, if we don't impose a quota then the first device that probes
> > > will get most/all of the MSIs and other devices miss out.
> > 
> > Out of curiosity - how pSeries has had done it without quotas before
> > 448e2ca ("powerpc/pseries: Implement a quota system for MSIs")?
> 
> Hmmm... do we need to treat this any differently?  If the platform
> can't allocate full range of requested MSIs, just failing should be
> enough regardless of why such allocation can't be met, no?

That depends from what "full range of requested MSIs" is. If that is
a maximum number of MSIs the controller advertised, then no. As MSI
design essentially allows devices to operate in lower-than-maximum
modes it is responsibility of a caller to decide how many vectors to
request. So in case of pSeries I think it is completely legitimate to
request lessers to overcome the platform limitation and let all devices
work.

> I really think what we should do is
> 
> * Determine the number of MSIs the controller wants.  Don't worry
>   about quotas or limits or anything.  Just determine the number
>   necessary to enable enhanced interrupt handling.

Actually, I do not see much contradiction with what I proposed. The
key words here "determine the number of MSIs the controller wants".

In general case it is not what pci_msix_table_size() returns (or at
least we should not limit ourselves to it) - there could be non-
standard means to report number of MSIs: hardcoded, version-dependant,
device-specific registers etc.

Next, if we opt to determine the number of MSIs by non-MSI standard
means then there is no reason not to call pci_get_msix_limit() (or
whatever) at this step.

The question how I see it - do we want pci_get_msix_limit() interface
as part of the MSI framework or do we want it pSeries-specific?

> * Try allocating that number of MSIs.  If it fails, then just revert
>   to single interrupt mode.  It's not the end of the world and mostly
>   guaranteed to work.  Let's please not even try to do partial
>   multiple interrupts.  I really don't think it's worth the risk or
>   complexity.

Being Captain Obvious here, but it is up to the device driver to handle
a failure. There could be no such option as single MSI mode after all :)

> Thanks.
> 
> -- 
> tejun

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