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@xxxxxxxxxx
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