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

[Bjorn Helgaas <helgaas@kernel.org>]

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.

> +#endif
> +
>  static void quirk_intel_th_dnv(struct pci_dev *dev)
>  {
>  	struct resource *r = &dev->resource[4];
> -- 
> 2.39.1
>