[PATCH kernel] powerpc/pci: Fix broken INTx configuration via OF

[Bjorn Helgaas]

On Thu, Feb 08, 2018 at 01:20:04PM -0600, Bjorn Helgaas wrote:
> [+cc linux-pci]
> 
> The original commit was merged via PCI, and I think it's a good idea
> to merge fixes to it the same way.  I'll try to merge this in time for
> v4.16-rc1.
> 
> On Wed, Feb 7, 2018 at 11:33 PM, Alexey Kardashevskiy <aik at ozlabs.ru> wrote:
> > Commit 59f47eff03a0 ("powerpc/pci: Use of_irq_parse_and_map_pci() helper")
> > correctly states that of_irq_parse_and_map_pci() does the same thing as
> > of_irq_parse_pci() does as it simply calls
> > of_irq_parse_pci() and irq_create_of_mapping().
> >
> > However of_irq_parse_and_map_pci() not only returns 0 for success and
> > negative value for an error but also a positive virq value from
> > irq_create_of_mapping() which the mentioned commit ignores and
> > INTx config fails.
> >
> > This fixes the of_irq_parse_and_map_pci() return value handling.
> >
> > Fixes: 59f47eff03a0 "powerpc/pci: Use of_irq_parse_and_map_pci() helper"
> > Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
> > ---
> >
> > Found it on POWER9 + powernv system - almost all devices suddenly lost
> > INTx support.
> > ---
> >  arch/powerpc/kernel/pci-common.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/kernel/pci-common.c b/arch/powerpc/kernel/pci-common.c
> > index ae2ede4..acbb44f2 100644
> > --- a/arch/powerpc/kernel/pci-common.c
> > +++ b/arch/powerpc/kernel/pci-common.c
> > @@ -370,7 +370,8 @@ static int pci_read_irq_line(struct pci_dev *pci_dev)
> >         memset(&oirq, 0xff, sizeof(oirq));
> >  #endif
> >         /* Try to get a mapping from the device-tree */
> > -       if (!of_irq_parse_and_map_pci(pci_dev, 0, 0)) {
> > +       virq = of_irq_parse_and_map_pci(pci_dev, 0, 0);
> > +       if (virq <= 0) {

I don't understand how this fix works.  We used to check the result of
of_irq_parse_and_map_pci() and entered the block if it was zero.

Now you enter the block if it is zero or less than zero, but:

  static int pci_read_irq_line(...)
  {
    unsigned int virq = 0;     /* unnecessarily initialized, BTW */

    virq = of_irq_parse_and_map_pci(pci_dev, 0, 0);
    if (virq <= 0) {
      ...

virq is unsigned, so "virq < 0" can never be true.  So how does this
change anything?

Bjorn