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

[Nirmal Patel <nirmal.patel@linux.intel.com>]

On Thu, 25 Jul 2024 09:40:13 +0530
Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org> wrote:

> 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
> 

Thanks for the response and suggestion. I will make adjustments in VMD
driver code.

-nirmal