Re: [RFC PATCH v4 03/20] PCI: Make pci_create_root_bus() declare its reliance on MSI domains

[Sunil V L]

Hi Bjorn,

On Mon, Apr 15, 2024 at 06:15:23PM -0500, Bjorn Helgaas wrote:
> On Mon, Apr 15, 2024 at 10:30:56PM +0530, Sunil V L wrote:
> > Similar to commit 9ec37efb8783 ("PCI/MSI: Make
> > pci_host_common_probe() declare its reliance on MSI domains"), declare
> > this dependency for PCI probe in ACPI based flow.
> > 
> > This is required especially for RISC-V platforms where MSI controller
> > can be absent. However, setting this for all architectures seem to cause
> > issues on non RISC-V architectures [1]. Hence, enabled this only for
> > RISC-V.
> > 
> > [1] - https://lore.kernel.org/oe-lkp/202403041047.791cb18e-oliver.sang@xxxxxxxxx
> > 
> > Signed-off-by: Sunil V L <sunilvl@xxxxxxxxxxxxxxxx>
> > ---
> >  drivers/pci/probe.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 1325fbae2f28..e09915bee2ee 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -3048,6 +3048,9 @@ struct pci_bus *pci_create_root_bus(struct device *parent, int bus,
> >  	bridge->sysdata = sysdata;
> >  	bridge->busnr = bus;
> >  	bridge->ops = ops;
> > +#ifdef CONFIG_RISCV
> > +	bridge->msi_domain = true;
> > +#endif
> 
> Ugh.  I looked at [1], but that's not a very good justification for
> this #ifdef.  The fault mentioned in [1] would need to be fixed, but
> not this way.
> 

Thank you again for the feedback!

I agree. This is due to my limitation with knowledge and resources to
debug the issue happening on non-UEFI x86 system with some particular
PCIe RC. Also, I was worried that we get into a rat hole of
assumptions/quirks with various architecture/PCIe RC combinations.

For ex: I think the issue is, somehow MSI domain is not set at the time
of PCI host bridge registration in pci_register_host_bridge() causing
PCI_BUS_FLAGS_NO_MSI to be set. This causes pci_alloc_irq_vectors() to
fail. In portdrv.c, pcie_init_service_irqs() doesn't switch to INTx
handling if MSI can not be used. It switches only if pcie_pme_no_msi()
returns true. I couldn't find who actually sets up MSI domain bit late
on this platform so that it somehow worked when we didn't set this flag.

Unfortunately, I don't have system to root cause and fix this issue with
confidence. Also, I don't know if any other architectures have similar
issues which are not caught yet. Hence, I thought it may be better
just restrict the change to RISC-V.

Let me know your thoughts. If there are better ways, I will be happy to
update.

Thanks,
Sunil