rust: add function to create masks

Create a function that is equivalent to Linux's GENMASK macro.

Signed-off-by: Fabien Parent <fabien.parent@linaro.org>

Author: Fabo

rust/kernel/bits.rs

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Bit manipulation helpers
+//!
+//! C header: [`include/linux/bits.h`](srctree/include/linux/bits.h)
+
+//use crate::static_assert;
+
+/// Generate a mask where all bits >= `h` and <= `l` are set
+///
+/// This is a re-implementation in rust of `GENMASK`
+pub const fn genmask(h: u32, l: u32) -> u32 {
+    //static_assert!(h >= l);
+    //static_assert!(h < 32);
+    ((!0u32) - (1 << l) + 1) & ((!0u32) >> (32 - 1 - h))
+}

rust/kernel/lib.rs

@@ -33,6 +33,7 @@ extern crate self as kernel;
 #[cfg(not(test))]
 #[cfg(not(testlib))]
 mod allocator;
+pub mod bits;
 mod build_assert;
 pub mod cred;
 pub mod device;