[PATCH v2 01/31] rust: pci: add domain_nr() accessor

The Rust PCI abstraction exposes a device's bus and device numbers
through dev_id(), which returns the packed PCI_DEVID value, but it does
not provide an accessor for the PCI domain number.

Add domain_nr(). Because pci_domain_nr() is a static inline rather than
an exported function, the accessor needs a helper as well.

The nova-core GPU driver needs the domain to build the domain/bus/device
word that GSP-RM firmware requires.

Assisted-by: Cursor:claude-opus-5
Signed-off-by: John Hubbard <[email protected]>
---
 rust/helpers/pci.c |  5 +++++
 rust/kernel/pci.rs | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/rust/helpers/pci.c b/rust/helpers/pci.c
index a714cc2bfb7a..e11ddb4de54e 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 287a4d259f13..c661829434a3 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -405,6 +405,18 @@ pub fn dev_id(&self) -> u16 {
         unsafe { bindings::pci_dev_id(self.as_raw()) }
     }
 
+    /// Returns the PCI domain number of the bus 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: The C function returns `int`, but a PCI domain number is always
+        // non-negative, so this cast will not lose any information.
+        domain_nr as u32
+    }
+
     /// Returns the PCI subsystem vendor ID.
     #[inline]
     pub fn subsystem_vendor_id(&self) -> u16 {
-- 
2.55.0