node: Put NodeType <-> dev_t conversions closer together

Group all this logic together in functions inside `impl NodeType`.

src/node/mod.rs

@@ -44,16 +44,7 @@ impl DrmNode {
             return Err(CreateDrmNodeError::NotDrmNode);
         }
 
-        // The type of the DRM node is determined by the minor number ranges:
-        //   0 -  63 -> Primary
-        //  64 - 127 -> Control
-        // 128 - 255 -> Render
-        let ty = match minor(dev) >> 6 {
-            0 => NodeType::Primary,
-            1 => NodeType::Control,
-            2 => NodeType::Render,
-            _ => return Err(CreateDrmNodeError::NotDrmNode),
-        };
+        let ty = NodeType::from_dev_id(dev)?;
 
         Ok(DrmNode { dev, ty })
     }
@@ -140,6 +131,12 @@ pub enum NodeType {
 }
 
 impl NodeType {
+    /// Bit-offset of [`NodeType`] inside [`minor()`]
+    const MINOR_OFFSET: u32 = 6;
+    /// Mask of [`NodeType`] inside [`minor()`]
+    #[cfg(not(target_os = "linux"))]
+    const MINOR_MASK: u32 = 0b11 << Self::MINOR_OFFSET;
+
     /// Returns a string representing the prefix of a minor device's name.
     ///
     /// For example, on Linux with a primary node, the returned string would be `card`.
@@ -151,14 +148,33 @@ impl NodeType {
         }
     }
 
+    fn from_dev_id(dev: dev_t) -> Result<Self, CreateDrmNodeError> {
+        // The type of the DRM node is determined by the minor number ranges:
+        //   0 -  63 -> Primary
+        //  64 - 127 -> Control
+        // 128 - 255 -> Render
+        Ok(match minor(dev) >> Self::MINOR_OFFSET {
+            0 => Self::Primary,
+            1 => Self::Control,
+            2 => Self::Render,
+            _ => return Err(CreateDrmNodeError::NotDrmNode),
+        })
+    }
+
     #[cfg(not(target_os = "linux"))]
-    fn minor_base(&self) -> u32 {
+    fn minor_index(&self) -> u32 {
         match self {
             NodeType::Primary => 0,
-            NodeType::Control => 64,
-            NodeType::Render => 128,
+            NodeType::Control => 1,
+            NodeType::Render => 2,
         }
     }
+
+    /// Returns the value to place at [`Self::MINOR_MASK`]
+    #[cfg(not(target_os = "linux"))]
+    fn minor_base(&self) -> u32 {
+        self.minor_index() << Self::MINOR_OFFSET
+    }
 }
 
 impl Display for NodeType {
@@ -330,8 +346,7 @@ pub fn dev_path(dev: dev_t, ty: NodeType) -> io::Result<PathBuf> {
     if let Some(dev_name) = devname(dev) {
         let suffix = dev_name.trim_start_matches(|c: char| !c.is_numeric());
         if let Ok(old_id) = suffix.parse::<u32>() {
-            const ID_MASK: u32 = 0b11_1111;
-            let id = old_id & ID_MASK | ty.minor_base();
+            let id = old_id & !NodeType::MINOR_MASK | ty.minor_base();
             let path = PathBuf::from(format!("/dev/dri/{}{}", ty.minor_name_prefix(), id));
             if path.exists() {
                 return Ok(path);
@@ -363,8 +378,7 @@ pub fn dev_path(dev: dev_t, ty: NodeType) -> io::Result<PathBuf> {
     }
 
     let old_id = minor(dev);
-    const ID_MASK: u32 = 0b11_1111;
-    let id = old_id & ID_MASK | ty.minor_base();
+    let id = old_id & !NodeType::MINOR_MASK | ty.minor_base();
     let path = PathBuf::from(format!("/dev/dri/{}{}", ty.minor_name_prefix(), id));
     if path.exists() {
         return Ok(path);