rust: dma: declare cfi_encoding for dma_data_direction

bindgen generates bindings::dma_data_direction as an integer by default,
while C callback tables such as dma_buf_ops declare the parameter as
enum dma_data_direction. kCFI encodes enum types by name even with
integer normalization, so the type ids would not match when C calls into
a Rust callback taking bindings::dma_data_direction.

Generate dma_data_direction as a newtype with a cfi_encoding, as is done
for lru_status, and convert DataDirection through the newtype's
constants.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Cc: stable@vger.kernel.org
Fixes: c2437c43cf ("rust: dma: implement DataDirection")
Link: https://patch.msgid.link/20261019-rust-cfi-fixes-v1-3-9503472ba738@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/bindgen_parameters

@@ -27,6 +27,8 @@
 --opaque-type spinlock
 
 # enums that appear in indirect function calls should specify a cfi type
+--newtype-enum dma_data_direction
+--with-attribute-custom-enum=dma_data_direction='#[cfi_encoding="18dma_data_direction"]'
 --newtype-enum hrtimer_restart
 --with-attribute-custom-enum=hrtimer_restart='#[cfi_encoding="15hrtimer_restart"]'
 --newtype-enum irqreturn

rust/kernel/dma.rs

@@ -310,25 +310,25 @@ pub enum DataDirection {
     ///
     /// This is used when the buffer can be both read from and written to by the device.
     /// The cache for the corresponding memory region is both flushed and invalidated.
-    Bidirectional = Self::const_cast(bindings::dma_data_direction_DMA_BIDIRECTIONAL),
+    Bidirectional = Self::const_cast(bindings::dma_data_direction::DMA_BIDIRECTIONAL),
 
     /// The DMA mapping is for data transfer from memory to the device (write).
     ///
     /// The CPU has prepared data in the buffer, and the device will read it.
     /// The cache for the corresponding memory region is flushed before device access.
-    ToDevice = Self::const_cast(bindings::dma_data_direction_DMA_TO_DEVICE),
+    ToDevice = Self::const_cast(bindings::dma_data_direction::DMA_TO_DEVICE),
 
     /// The DMA mapping is for data transfer from the device to memory (read).
     ///
     /// The device will write data into the buffer for the CPU to read.
     /// The cache for the corresponding memory region is invalidated before CPU access.
-    FromDevice = Self::const_cast(bindings::dma_data_direction_DMA_FROM_DEVICE),
+    FromDevice = Self::const_cast(bindings::dma_data_direction::DMA_FROM_DEVICE),
 
     /// The DMA mapping is not for data transfer.
     ///
     /// This is primarily for debugging purposes. With this direction, the DMA mapping API
     /// will not perform any cache coherency operations.
-    None = Self::const_cast(bindings::dma_data_direction_DMA_NONE),
+    None = Self::const_cast(bindings::dma_data_direction::DMA_NONE),
 }
 
 impl DataDirection {
@@ -340,7 +340,7 @@ const fn const_cast(val: bindings::dma_data_direction) -> u32 {
         // CAST: The C standard allows compilers to choose different integer types for enums.
         // To safely check the value, we cast it to a wide signed integer type (`i128`)
         // which can hold any standard C integer enum type without truncation.
-        let wide_val = val as i128;
+        let wide_val = val.0 as i128;
 
         // Check if the value is outside the valid range for the target type `u32`.
         // CAST: `u32::MAX` is cast to `i128` to match the type of `wide_val` for the comparison.
@@ -358,11 +358,12 @@ const fn const_cast(val: bindings::dma_data_direction) -> u32 {
 impl From<DataDirection> for bindings::dma_data_direction {
     /// Returns the raw representation of [`enum dma_data_direction`].
     fn from(direction: DataDirection) -> Self {
-        // CAST: `direction as u32` gets the underlying representation of our `#[repr(u32)]` enum.
-        // The subsequent cast to `Self` (the bindgen type) assumes the C enum is compatible
-        // with the enum variants of `DataDirection`, which is a valid assumption given our
-        // compile-time checks.
-        direction as u32 as Self
+        match direction {
+            DataDirection::Bidirectional => Self::DMA_BIDIRECTIONAL,
+            DataDirection::ToDevice => Self::DMA_TO_DEVICE,
+            DataDirection::FromDevice => Self::DMA_FROM_DEVICE,
+            DataDirection::None => Self::DMA_NONE,
+        }
     }
 }