[PATCH 1/2] PCI: Provide sensible irq vector alloc/free routines

[Alexander Gordeev]

On Thu, May 12, 2016 at 01:03:18PM +0200, Christoph Hellwig wrote:
> > Okay, now we have a subtlety here. You switch from MSI-X to MSI in case
> > MSI-X retries exhausted. At this point nr_vecs is lowest (likely 1). So
> > you start trying MSI from 1 rather from 32. I would suggest two subsequent
> > loops instead.
> 
> So bridges could support less MSI-X entries than MSI ones?  Yikes.

Not this way. MSI vectors could be a scarce resource in a platform. So
even though devices could support more MSI-Xs than MSIs the underlying
platform might fail to fulfil a device request. 

> > Another thing, pci_alloc_irq_vectors() tries to allocate vectors in a
> > range from 1 to nr_vecs now. So this function implicitly falls into
> > the other two range functions family and therefore:
> > 
> >   (a) pci_alloc_irq_vectors() name is not perfec;
> 
> What would you call it instead?

I do not know, really :( I would expect "range" within the name since
a range requested indeed, but I am just hinting here.

> >   (b) why not introduce 'minvec' minimal number of interrupts then?
> >       We could have a handy pci_enable_irq_range() as result;
> 
> That seems pretty pointless, when the caller can simply treat a too
> small number as failure and use the existing failure path for that.

There was a huge discussion on this few years ago, when the range
functions were introduced. Actually, the prototypes of these two is
the outcome of that discussion. I almost sure your point was expressed
by many at the time ;)

> >   (c) if you do (b) then PCI_IRQ_NOMSIX flag becomes redundant, since
> >       caller would invoke pci_enable_msi_range() instead;
> 
> pci_enable_msi_range still has a horrible API that forces the caller
> to deal with the irq numbers differently than the MSI-X case, so it
> should also go away in the long run.

Well, if we introduce pci_enable_irq_range() (or something) and
pci_get_dev_irq(int vector) (or something) that covers MSI-X, MSI and
legacy IRQs then we can get it done now. Your pci_alloc_irq_vectors()
is just few steps from there, huh?

(Sorry for the weird quotting)