[PATCH v3 13/14] PCI/MSI: Document the various ways of ending up with NO_MSI

[Jeremy Linton]

Hi,


On 3/30/21 10:11 AM, Marc Zyngier wrote:
> We have now three ways of ending up with NO_MSI being set.
> Document them.
> 
> Acked-by: Bjorn Helgaas <bhelgaas at google.com>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>   drivers/pci/msi.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> index d9c73c173c14..217dc9f0231f 100644
> --- a/drivers/pci/msi.c
> +++ b/drivers/pci/msi.c
> @@ -871,8 +871,15 @@ static int pci_msi_supported(struct pci_dev *dev, int nvec)
>   	 * Any bridge which does NOT route MSI transactions from its
>   	 * secondary bus to its primary bus must set NO_MSI flag on
>   	 * the secondary pci_bus.
> -	 * We expect only arch-specific PCI host bus controller driver
> -	 * or quirks for specific PCI bridges to be setting NO_MSI.
> +	 *
> +	 * The NO_MSI flag can either be set directly by:
> +	 * - arch-specific PCI host bus controller drivers (deprecated)
> +	 * - quirks for specific PCI bridges
> +	 *
> +	 * or indirectly by platform-specific PCI host bridge drivers by
> +	 * advertising the 'msi_domain' property, which results in
> +	 * the NO_MSI flag when no MSI domain is found for this bridge
> +	 * at probe time.

I have an ACPI machine with a gicv2 (no m), and a MSI region that isn't 
described by ACPI because its non-standard. In the past this tended to 
work because PCIe device drivers would fall back to legacy pci intx 
silently. But, with 5.13, it seems this series now triggers the 
WARN_ON_ONCE() in arch_setup_msi_irq, because duh, no MSI support.

Everything of course continues to work, it just gets this ugly splat on 
bootup telling me basically the machine doesn't support MSIs. So, I 
considered a few patches, including just basically setting nomsi if 
gicv2 && acpi, or eek a host bridge quirk.

None of these seem great, so how can this be fixed?

Thanks,




>   	 */
>   	for (bus = dev->bus; bus; bus = bus->parent)
>   		if (bus->bus_flags & PCI_BUS_FLAGS_NO_MSI)
>