[PATCH v3 3/5] 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.

Cc: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Zhi Wang <zhiw@nvidia.com>
---
 rust/kernel/pci.rs | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index 77a8eb39ad32..9ebba8e08d2e 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -418,6 +418,19 @@ fn as_raw(&self) -> *mut bindings::pci_dev {
     }
 }
 
+/// Represents the size of a PCI configuration space.
+///
+/// PCI devices can have either a *normal* (legacy) configuration space of 256 bytes,
+/// or an *extended* configuration space of 4096 bytes as defined in the PCI Express
+/// specification.
+pub enum ConfigSpaceSize {
+    /// 256-byte legacy PCI configuration space.
+    Normal = 256,
+
+    /// 4096-byte PCIe extended configuration space.
+    Extended = 4096,
+}
+
 impl Device {
     /// Returns the PCI vendor ID as [`Vendor`].
     ///
@@ -514,6 +527,17 @@ pub fn pci_class(&self) -> Class {
         // SAFETY: `self.as_raw` is a valid pointer to a `struct pci_dev`.
         Class::from_raw(unsafe { (*self.as_raw()).class })
     }
+
+    /// Returns the size of configuration space.
+    pub fn cfg_size(&self) -> Result<ConfigSpaceSize> {
+        // SAFETY: `self.as_raw` is a valid pointer to a `struct pci_dev`.
+        let size = unsafe { (*self.as_raw()).cfg_size };
+        match size {
+            256 => Ok(ConfigSpaceSize::Normal),
+            4096 => Ok(ConfigSpaceSize::Extended),
+            _ => Err(EINVAL),
+        }
+    }
 }
 
 impl Device<device::Bound> {
-- 
2.47.3