Re: [RFC PATCH v2 13/21] irqchip: riscv-intc: Add ACPI support for AIA

[Sunil V L]

Hi Bjorn,

On Thu, Oct 26, 2023 at 11:51:50AM -0500, Bjorn Helgaas wrote:
> On Thu, Oct 26, 2023 at 01:53:36AM +0530, Sunil V L wrote:
> > The RINTC subtype structure in MADT also has information about other
> > interrupt controllers like MMIO. So, save those information and provide
> > interfaces to retrieve them when required by corresponding drivers.
> 
> > @@ -218,7 +306,19 @@ static int __init riscv_intc_acpi_init(union acpi_subtable_headers *header,
> 
> > +	 * MSI controller (IMSIC) in RISC-V is optional. So, unless
> > +	 * IMSIC is discovered, set system wide MSI support as
> > +	 * unsupported. Once IMSIC is probed, MSI support will be set.
> > +	 */
> > +	pci_no_msi();
> 
> It doesn't seem like we should have to tell the PCI core about
> functionality we *don't* have.
> 
> I would think IMSIC would be detected before enumerating PCI devices
> that might use it, and if we *haven't* found an IMSIC by the time we
> get to pci_register_host_bridge(), would/should we set
> PCI_BUS_FLAGS_NO_MSI there?
>
The check in pci_register_host_bridge() is like below.

if (bridge->msi_domain && !dev_get_msi_domain(&bus->dev) &&
            !pci_host_of_has_msi_map(parent))
                bus->bus_flags |= PCI_BUS_FLAGS_NO_MSI;

When there is no IMSIC, bridge->msi_domain is 0 and hence
PCI_BUS_FLAGS_NO_MSI will never be set. Do you recommend to set
PCI_BUS_FLAGS_NO_MSI if bridge->msi_domain is 0? Let me know if I am
missing something.

Thanks,
Sunil