[PATCH v5 6/7] rust: pci: add config space read/write support

[Zhi Wang <zhiw@nvidia.com>]

Drivers might need to access PCI config space for querying capability
structures and access the registers inside the structures.

For Rust drivers need to access PCI config space, the Rust PCI abstraction
needs to support it in a way that upholds Rust's safety principles.

Introduce a `ConfigSpace` wrapper in Rust PCI abstraction to provide safe
accessors for PCI config space. The new type implements the `Io` trait to
share offset validation and bound-checking logic with others.

Cc: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Zhi Wang <zhiw@nvidia.com>
---
 rust/kernel/pci.rs    | 55 +++++++++++++++++++++++++++++++++++++-
 rust/kernel/pci/io.rs | 62 ++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 115 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
index 410b79d46632..0b8064d6c0d1 100644
--- a/rust/kernel/pci.rs
+++ b/rust/kernel/pci.rs
@@ -39,7 +39,10 @@
     ClassMask,
     Vendor, //
 };
-pub use self::io::Bar;
+pub use self::io::{
+    Bar,
+    ConfigSpace, //
+};
 pub use self::irq::{
     IrqType,
     IrqTypes,
@@ -330,6 +333,28 @@ 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.
+#[repr(usize)]
+pub enum ConfigSpaceSize {
+    /// 256-byte legacy PCI configuration space.
+    Normal = 256,
+
+    /// 4096-byte PCIe extended configuration space.
+    Extended = 4096,
+}
+
+impl ConfigSpaceSize {
+    /// Get the raw value of this enum.
+    #[inline(always)]
+    pub const fn as_raw(self) -> usize {
+        self as usize
+    }
+}
+
 impl Device {
     /// Returns the PCI vendor ID as [`Vendor`].
     ///
@@ -426,6 +451,20 @@ 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.
+    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),
+            _ => {
+                debug_assert!(false);
+                Err(EINVAL)
+            }
+        }
+    }
 }
 
 impl Device<device::Core> {
@@ -441,6 +480,20 @@ pub fn set_master(&self) {
         // SAFETY: `self.as_raw` is guaranteed to be a pointer to a valid `struct pci_dev`.
         unsafe { bindings::pci_set_master(self.as_raw()) };
     }
+
+    /// Return an initialized config space object.
+    pub fn config_space<'a>(
+        &'a self,
+    ) -> Result<ConfigSpace<'a, { ConfigSpaceSize::Normal.as_raw() }>> {
+        Ok(ConfigSpace { pdev: self })
+    }
+
+    /// Return an initialized config space object.
+    pub fn config_space_exteneded<'a>(
+        &'a self,
+    ) -> Result<ConfigSpace<'a, { ConfigSpaceSize::Extended.as_raw() }>> {
+        Ok(ConfigSpace { pdev: self })
+    }
 }
 
 // SAFETY: `Device` is a transparent wrapper of a type that doesn't depend on `Device`'s generic
diff --git a/rust/kernel/pci/io.rs b/rust/kernel/pci/io.rs
index 2bbb3261198d..a9895ee2c5c8 100644
--- a/rust/kernel/pci/io.rs
+++ b/rust/kernel/pci/io.rs
@@ -2,12 +2,19 @@
 
 //! PCI memory-mapped I/O infrastructure.
 
-use super::Device;
+use super::{
+    Device,
+    ConfigSpaceSize, //
+};
 use crate::{
     bindings,
     device,
     devres::Devres,
     io::{
+        define_read,
+        define_write,
+        Io,
+        IoInfallible,
         Mmio,
         MmioRaw, //
     },
@@ -16,6 +23,59 @@
 };
 use core::ops::Deref;
 
+/// Represents the PCI configuration space of a device.
+///
+/// Provides typed read and write accessors for configuration registers
+/// using the standard `pci_read_config_*` and `pci_write_config_*` helpers.
+///
+/// The generic const parameter `SIZE` can be used to indicate the
+/// maximum size of the configuration space (e.g. 256 bytes for legacy,
+/// 4096 bytes for extended config space). The actual size is obtained
+/// from the underlying `struct pci_dev` via [`Device::cfg_size`].
+pub struct ConfigSpace<'a, const SIZE: usize = { ConfigSpaceSize::Extended as usize }> {
+    pub(crate) pdev: &'a Device<device::Core>,
+}
+
+macro_rules! call_config_read {
+    (infallible, $c_fn:ident, $self:ident, $ty:ty, $addr:expr) => {{
+        let mut val: $ty = 0;
+        let _ret = unsafe { bindings::$c_fn($self.pdev.as_raw(), $addr as i32, &mut val) };
+        val
+    }};
+}
+
+macro_rules! call_config_write {
+    (infallible, $c_fn:ident, $self:ident, $ty:ty, $addr:expr, $value:expr) => {
+        let _ret = unsafe { bindings::$c_fn($self.pdev.as_raw(), $addr as i32, $value) };
+    };
+}
+
+impl<'a, const SIZE: usize> Io for ConfigSpace<'a, SIZE> {
+    const MIN_SIZE: usize = SIZE;
+
+    /// Returns the base address of this mapping.
+    #[inline]
+    fn addr(&self) -> usize {
+        0
+    }
+
+    /// Returns the maximum size of this mapping.
+    #[inline]
+    fn maxsize(&self) -> usize {
+        self.pdev.cfg_size().map_or(0, |v| v as usize)
+    }
+}
+
+impl<'a, const SIZE: usize> IoInfallible for ConfigSpace<'a, SIZE> {
+    define_read!(infallible, read8, call_config_read, pci_read_config_byte -> u8);
+    define_read!(infallible, read16, call_config_read, pci_read_config_word -> u16);
+    define_read!(infallible, read32, call_config_read, pci_read_config_dword -> u32);
+
+    define_write!(infallible, write8, call_config_write, pci_write_config_byte <- u8);
+    define_write!(infallible, write16, call_config_write, pci_write_config_word <- u16);
+    define_write!(infallible, write32, call_config_write, pci_write_config_dword <- u32);
+}
+
 /// A PCI BAR to perform I/O-Operations on.
 ///
 /// # Invariants
-- 
2.51.0