Add support for Memory Reservations in FDT

Author: m4tx

Cargo.toml

@@ -17,7 +17,7 @@ alloc = []
 std = ["alloc"]
 
 [dependencies]
-zerocopy = { version = "0.8", features = ["derive"] }
+zerocopy = { version = "0.8.28", features = ["derive"] }
 
 [lints.rust]
 deprecated-safe = "warn"

src/error.rs

@@ -43,6 +43,11 @@ pub enum FdtErrorKind {
     InvalidOffset,
     /// An invalid string was encountered.
     InvalidString,
+    /// Memory reservation block has not been terminated with a null entry.
+    MemReserveNotTerminated,
+    /// Memory reservation block has an entry that is unaligned or has invalid
+    /// size.
+    MemReserveInvalid,
 }
 
 impl fmt::Display for FdtError {
@@ -65,6 +70,14 @@ impl fmt::Display for FdtErrorKind {
             FdtErrorKind::BadToken(token) => write!(f, "bad FDT token: 0x{token:x}"),
             FdtErrorKind::InvalidOffset => write!(f, "invalid offset in FDT"),
             FdtErrorKind::InvalidString => write!(f, "invalid string in FDT"),
+            FdtErrorKind::MemReserveNotTerminated => write!(
+                f,
+                "memory reservation block not terminated with a null entry"
+            ),
+            FdtErrorKind::MemReserveInvalid => write!(
+                f,
+                "memory reservation block has an entry that is unaligned or has invalid size"
+            ),
         }
     }
 }

src/fdt/mod.rs

@@ -16,6 +16,7 @@
 //! [Flattened Device Tree (FDT)]: https://devicetree-specification.readthedocs.io/en/latest/chapter5-flattened-format.html
 
 use crate::error::{FdtError, FdtErrorKind};
+use crate::memreserve::MemoryReservation;
 mod node;
 mod property;
 use core::ffi::CStr;
@@ -312,6 +313,34 @@ impl<'a> Fdt<'a> {
         self.header().boot_cpuid_phys()
     }
 
+    /// Returns an iterator over the memory reservation block.
+    pub fn memory_reservations(
+        &self,
+    ) -> impl Iterator<Item = Result<MemoryReservation, FdtError>> + '_ {
+        let mut offset = self.header().off_mem_rsvmap() as usize;
+        core::iter::from_fn(move || {
+            if offset >= self.header().off_dt_struct() as usize {
+                return Some(Err(FdtError::new(
+                    FdtErrorKind::MemReserveNotTerminated,
+                    offset,
+                )));
+            }
+
+            let reservation = match MemoryReservation::ref_from_prefix(&self.data[offset..])
+                .map_err(|_| FdtError::new(FdtErrorKind::MemReserveInvalid, offset))
+            {
+                Ok((reservation, _)) => *reservation,
+                Err(e) => return Some(Err(e)),
+            };
+            offset += size_of::<MemoryReservation>();
+
+            if reservation == MemoryReservation::TERMINATOR {
+                return None;
+            }
+            Some(Ok(reservation))
+        })
+    }
+
     /// Returns the root node of the device tree.
     ///
     /// # Errors
@@ -498,6 +527,15 @@ impl<'a> Fdt<'a> {
 impl fmt::Display for Fdt<'_> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         writeln!(f, "/dts-v1/;")?;
+        for reservation in self.memory_reservations() {
+            let reservation = reservation.map_err(|_| fmt::Error)?;
+            writeln!(
+                f,
+                "/memreserve/ {:#x} {:#x};",
+                reservation.address(),
+                reservation.size()
+            )?;
+        }
         writeln!(f)?;
         let root = self.root().map_err(|_| fmt::Error)?;
         root.fmt_recursive(f, 0)

src/lib.rs

@@ -23,5 +23,7 @@
 #![warn(missing_docs, rustdoc::missing_crate_level_docs)]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
+pub mod memreserve;
+
 pub mod error;
 pub mod fdt;

src/memreserve.rs

@@ -0,0 +1,57 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Device tree memory reservations.
+
+use zerocopy::{FromBytes, Immutable, IntoBytes, KnownLayout, big_endian};
+
+/// A 64-bit memory reservation.
+#[derive(
+    Debug,
+    Clone,
+    Copy,
+    PartialEq,
+    Eq,
+    PartialOrd,
+    Ord,
+    Hash,
+    FromBytes,
+    IntoBytes,
+    Immutable,
+    KnownLayout,
+)]
+#[repr(C)]
+pub struct MemoryReservation {
+    address: big_endian::U64,
+    size: big_endian::U64,
+}
+
+impl MemoryReservation {
+    pub(crate) const TERMINATOR: Self = Self::new(0, 0);
+
+    /// Creates a new [`MemoryReservation`].
+    #[must_use]
+    pub const fn new(address: u64, size: u64) -> Self {
+        Self {
+            address: big_endian::U64::new(address),
+            size: big_endian::U64::new(size),
+        }
+    }
+
+    /// Returns the physical address of the reserved memory region.
+    #[must_use]
+    pub const fn address(&self) -> u64 {
+        self.address.get()
+    }
+
+    /// Returns the size of the reserved memory region.
+    #[must_use]
+    pub const fn size(&self) -> u64 {
+        self.size.get()
+    }
+}

tests/dtb/test_memreserve.dtb

@@ -0,0 +1,7 @@
+/dts-v1/;
+
+/memreserve/ 0x1000 0x100;
+/memreserve/ 0x2000 0x200;
+
+/ {
+};

tests/dts/test_memreserve.dts

@@ -0,0 +1,30 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use dtoolkit::fdt::Fdt;
+use dtoolkit::memreserve::MemoryReservation;
+
+#[test]
+fn memreserve() {
+    let dtb = include_bytes!("dtb/test_memreserve.dtb");
+    let fdt = Fdt::new(dtb).unwrap();
+
+    let reservations: Result<Vec<_>, _> = fdt.memory_reservations().collect();
+    let reservations = reservations.unwrap();
+    assert_eq!(
+        reservations,
+        &[
+            MemoryReservation::new(0x1000, 0x100),
+            MemoryReservation::new(0x2000, 0x200)
+        ]
+    );
+
+    let dts = fdt.to_string();
+    assert!(dts.contains("/memreserve/ 0x1000 0x100;"));
+    assert!(dts.contains("/memreserve/ 0x2000 0x200;"));
+}