# Re: \[PATCH v2 12/15\] PCI/MSI: Let PCI host bridges declare their reliance on MSI domains

- From: Robin Murphy \<robin.murphy@arm.com\>
- Date: 2021-03-23 11:45:52 
- List: linux-mediatek
- Page: https://ratatoskr.run/linux-mediatek/2021/03/6312067
- Thread: https://ratatoskr.run/linux-pci/2021/03/6311157/t.md

```text
On 2021-03-22 18:46, Marc Zyngier wrote:
> The new 'no_msi' attribute solves the problem of advertising the lack
> of MSI capability for host bridges that know for sure that there will
> be no MSI for their end-points.
>
> However, there is a whole class of host bridges that cannot know
> whether MSIs will be provided or not, as they rely on other blocks
> to provide the MSI functionnality, using MSI domains.  This is
> the case for example on systems that use the ARM GIC architecture.
>
> Introduce a new attribute ('msi_domain') indicating that implicit
> dependency, and use this property to set the NO_MSI flag when
> no MSI domain is found at probe time.
>
> Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>   drivers/pci/probe.c | 2 +-
>   include/linux/pci.h | 1 +
>   2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 146bd85c037e..bac9f69a06a8 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -925,7 +925,7 @@ static int pci_register_host_bridge(struct pci_host_bridge *bridge)
>   	device_enable_async_suspend(bus->bridge);
>   	pci_set_bus_of_node(bus);
>   	pci_set_bus_msi_domain(bus);
> -	if (bridge->no_msi)
> +	if (bridge->no_msi || (bridge->msi_domain && !bus->dev.msi_domain))
>   		bus->bus_flags |= PCI_BUS_FLAGS_NO_MSI;
>   
>   	if (!parent)
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 48605cca82ae..d322d00db432 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -551,6 +551,7 @@ struct pci_host_bridge {
>   	unsigned int	preserve_config:1;	/* Preserve FW resource setup */
>   	unsigned int	size_windows:1;		/* Enable root bus sizing */
>   	unsigned int	no_msi:1;		/* Bridge has no MSI support */
> +	unsigned int	msi_domain:1;		/* Bridge wants MSI domain */

Aren't these really the same thing? Either way we're saying the bridge itself doesn't handle MSIs, it's just in one case we're effectively encoding a platform-specific assumption that an external domain won't be provided. I can't help wondering whether that distinction is really necessary...

Robin.

>   
>   	/* Resource alignment requirements */
>   	resource_size_t (*align_resource)(struct pci_dev *dev,
>

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