[RFC 3/6] rust: pci: add a helper to query configuration space size

[Zhi Wang <zhiw@nvidia.com>]

Expose a safe Rust wrapper for the `cfg_size` field of `struct pci_dev`,
allowing drivers to query the size of a device's configuration space.

This is useful for code that needs to know whether the device supports
extended configuration space (e.g. 256 vs 4096 bytes) when accessing PCI
configuration registers and apply runtime checks.

Signed-off-by: Zhi Wang <zhiw@nvidia.com>
---
 rust/kernel/pci.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index 887ee611b553..7a107015e7d2 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -410,6 +410,12 @@ pub fn resource_len(&self, bar: u32) -> Result<bindings::resource_size_t> {
         // - by its type invariant `self.as_raw` is always a valid pointer to a `struct pci_dev`.
         Ok(unsafe { bindings::pci_resource_len(self.as_raw(), bar.try_into()?) })
     }
+
+    /// Returns the size of configuration space.
+    pub fn cfg_size(&self) -> i32 {
+        // SAFETY: `self.as_raw` is a valid pointer to a `struct pci_dev`.
+        unsafe { (*self.as_raw()).cfg_size }
+    }
 }
 
 impl Device<device::Bound> {
-- 
2.47.3