[PATCH 1/2] rust: pci: add domain_nr() accessor

[John Hubbard <jhubbard@nvidia.com>]

The nova-core driver has to send the GPU's PCI location to the GPU's
firmware as one word that holds the domain, the bus and the device
number.

The PCI abstraction has dev_id(), which packs the bus, the device and
the function into one value. It has no accessor that reports the domain.

Add an accessor for the domain number.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 rust/helpers/pci.c |  5 +++++
 rust/kernel/pci.rs | 14 ++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/rust/helpers/pci.c b/rust/helpers/pci.c
index a8540f90b13c..49f2c089d236 100644
--- a/rust/helpers/pci.c
+++ b/rust/helpers/pci.c
@@ -7,6 +7,11 @@ __rust_helper u16 rust_helper_pci_dev_id(struct pci_dev *dev)
 	return PCI_DEVID(dev->bus->number, dev->devfn);
 }
 
+__rust_helper int rust_helper_pci_domain_nr(struct pci_dev *dev)
+{
+	return pci_domain_nr(dev->bus);
+}
+
 __rust_helper resource_size_t
 rust_helper_pci_resource_start(struct pci_dev *pdev, int bar)
 {
diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index 769d01d23429..57718e48208d 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -408,6 +408,20 @@ pub fn dev_id(&self) -> u16 {
         unsafe { bindings::pci_dev_id(self.as_raw()) }
     }
 
+    /// Returns the PCI domain number of the bus that this device is on.
+    #[inline]
+    pub fn domain_nr(&self) -> u32 {
+        // SAFETY: By its type invariant `self.as_raw` is always a valid pointer to a
+        // `struct pci_dev`.
+        let domain_nr = unsafe { bindings::pci_domain_nr(self.as_raw()) };
+
+        // CAST: `pci_domain_nr()` returns an `int`, but a registered bus never has a negative
+        // domain, because `pci_register_host_bridge()` fails when the domain allocation fails.
+        // A negative value here is a kernel bug rather than a condition for a caller to handle.
+        debug_assert!(domain_nr >= 0);
+        domain_nr as u32
+    }
+
     /// Returns the PCI subsystem vendor ID.
     #[inline]
     pub fn subsystem_vendor_id(&self) -> u16 {
-- 
2.56.0