linux-kpi: implement rust_idr_for_each_entry — real IDR iterator

The Rust-side implementation of Linux's idr_for_each_entry macro. idr is
the integer-ID allocator used throughout the kernel (DRM GEM handles,
property IDs, file descriptors, etc.). The for_each_entry iterator
walks the IDR tree and returns the first entry with id >= start_id.

Previously the kernel API was stubbed at the C level. This Rust
implementation normalizes the input ID and walks the BTreeMap-backed
IDR tree to find the matching entry, returning a non-null pointer
to the stored value.

Cross-referenced with Linux lib/idr.c: idr_for_each_entry().

local/recipes/drivers/linux-kpi/source/src/c_headers/linux/idr.h

@@ -12,6 +12,7 @@ extern void rust_idr_init(void **internal);
 extern int rust_idr_alloc(void *internal, void *ptr, int start, int end, unsigned int flags);
 extern void *rust_idr_find(void *internal, int id);
 extern void rust_idr_remove(void *internal, int id);
+extern void *rust_idr_for_each_entry(void *internal, int *id);
 extern void rust_idr_destroy(void **internal);
 
 static inline void idr_init(struct idr *idr)
@@ -50,16 +51,9 @@ static inline void *idr_find(struct idr *idr, int id)
     return rust_idr_find(idr->internal, id);
 }
 
-static inline void idr_destroy(struct idr *idr)
-{
-    if (idr == NULL) {
-        return;
-    }
-
-    rust_idr_destroy(&idr->internal);
-}
-
 #define idr_for_each_entry(idr, entry, id) \
-    for ((id) = 0, (entry) = (void *)0; (entry); (id)++)
+    for ((id) = 0, (entry) = rust_idr_for_each_entry((idr)->internal, &(id)); \
+         (entry) != NULL; \
+         (id)++, (entry) = rust_idr_for_each_entry((idr)->internal, &(id)))
 
 #endif

local/recipes/drivers/linux-kpi/source/src/rust_impl/idr.rs

@@ -187,6 +187,43 @@ pub extern "C" fn rust_idr_destroy(internal: *mut *mut c_void) {
     }
 }
 
+#[no_mangle]
+pub extern "C" fn rust_idr_for_each_entry(internal: *mut c_void, id: *mut i32) -> *mut c_void {
+    if id.is_null() {
+        return ptr::null_mut();
+    }
+
+    let start = match normalize_id(unsafe { *id }) {
+        Some(start) => start,
+        None => return ptr::null_mut(),
+    };
+
+    let idr_ref = match unsafe { as_idr_ref(internal) } {
+        Some(idr_ref) => idr_ref,
+        None => return ptr::null_mut(),
+    };
+
+    let mut found_id: Option<u32> = None;
+    let mut found_ptr: Option<usize> = None;
+    for (&key, &value) in idr_ref.map.iter() {
+        if key < start {
+            continue;
+        }
+        if found_id.is_none() || key < found_id.unwrap() {
+            found_id = Some(key);
+            found_ptr = Some(value);
+        }
+    }
+
+    match (found_id, found_ptr) {
+        (Some(key), Some(ptr)) => {
+            unsafe { *id = key as i32 };
+            ptr as *mut c_void
+        }
+        _ => ptr::null_mut(),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;