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

[Marc Zyngier <maz@kernel.org>]

Hi Robin,

On Tue, 23 Mar 2021 11:45:02 +0000,
Robin Murphy <robin.murphy@arm.com> wrote:
> 
> 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...

There is a subtle difference: no_msi indicates that there is no way
*any* MSI can be dealt with whatsoever (maybe because the RC doesn't
forward the corresponding TLPs?). msi_domain says "no MSI unless...".

We could implement the former with the latter, but I have the feeling
that's not totally bullet proof. Happy to revisit this if you think it
really matters.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.