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

[Lorenzo Pieralisi]

On Tue, Mar 23, 2021 at 06:09:36PM +0000, Marc Zyngier wrote:
> Hi Robin,
> 
> On Tue, 23 Mar 2021 11:45:02 +0000,
> Robin Murphy <robin.murphy@xxxxxxx> 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@xxxxxxxxxx>
> > > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> > > ---
> > >   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.

IIUC msi_domain == 1 means: this host bridge needs an msi_domain to enable
MSIs, which in turn means that there are bridges that do _not_ require
an msi_domain to enable MSIs. I don't know how other arches handle the 
msi_domain pointer but I am asking whether making:

if (bridge->no_msi || !bus->dev.msi_domain))
	bus->bus_flags |= PCI_BUS_FLAGS_NO_MSI;

is a possibility (removing the need for the msi_domain flag).

At least this looks more like an arch property than a host bridge
specific property (eg patch [13] pci_host_common_probe() may be used on
arches other than ARM where it is not necessary true that it requires an
msi_domain to enable MSIs).

I agree that's complicated to untangle - just asking if there is way
to simplify it.

Thanks,
Lorenzo

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