Re: [PATCH] PCI: fixup PCI_INTERRUPT_LINE for VMD downstream devices

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Wed, Jul 24, 2024 at 02:10:30PM -0500, Bjorn Helgaas wrote:
> On Wed, Jul 24, 2024 at 10:00:40AM -0700, Nirmal Patel wrote:
> > VMD does not support legacy interrupts for devices downstream from a
> > VMD endpoint. So initialize the PCI_INTERRUPT_LINE to 0 for these
> > devices to ensure we don't try to set up a legacy irq for them.
> 
> s/legacy interrupts/INTx/
> s/legacy irq/INTx/
> 
> > Note: This patch was proposed by Jim, I am trying to upstream it.
> > 
> > Signed-off-by: Jim Harris <james.r.harris@intel.com>
> > Signed-off-by: Nirmal Patel <nirmal.patel@linux.intel.com>
> > ---
> >  arch/x86/pci/fixup.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/arch/x86/pci/fixup.c b/arch/x86/pci/fixup.c
> > index b33afb240601..a3b34a256e7f 100644
> > --- a/arch/x86/pci/fixup.c
> > +++ b/arch/x86/pci/fixup.c
> > @@ -653,6 +653,20 @@ static void quirk_no_aersid(struct pci_dev *pdev)
> >  DECLARE_PCI_FIXUP_CLASS_EARLY(PCI_VENDOR_ID_INTEL, PCI_ANY_ID,
> >  			      PCI_CLASS_BRIDGE_PCI, 8, quirk_no_aersid);
> >  
> > +#if IS_ENABLED(CONFIG_VMD)
> > +/* 
> > + * VMD does not support legacy interrupts for downstream devices.
> > + * So PCI_INTERRPUT_LINE needs to be initialized to 0 to ensure OS
> > + * doesn't try to configure a legacy irq.
> 
> s/legacy interrupts/INTx/
> s/PCI_INTERRPUT_LINE/PCI_INTERRUPT_LINE/
> 
> > + */
> > +static void quirk_vmd_interrupt_line(struct pci_dev *dev)
> > +{
> > +	if (is_vmd(dev->bus))
> > +		pci_write_config_byte(dev, PCI_INTERRUPT_LINE, 0);
> > +}
> > +DECLARE_PCI_FIXUP_HEADER(PCI_ANY_ID, PCI_ANY_ID, quirk_vmd_interrupt_line);
> 
> A quirk for every PCI device, even on systems without VMD, seems like
> kind of a clumsy way to deal with this.
> 
> Conceptually, I would expect a host bridge driver (VMD acts like a
> host bridge in this case) to know whether it supports INTx, and if the
> driver knows it doesn't support INTx or it has no _PRT or DT
> description of INTx routing to use, an attempt to configure INTx
> should just fail naturally.
> 
> I don't claim this is how host bridge drivers actually work; I just
> think it's the way they *should* work.
> 

Absolutely! This patch is fixing the issue in a wrong place. There are existing
DT based host bridge drivers that disable INTx due to lack of hardware
capability. The driver just need to nullify pci_host_bridge::map_irq callback.

- Mani

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