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 [off-list ref] wrote:

quoted

On 2021-03-22 18:46, Marc Zyngier wrote:

quoted

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.

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.