rust: block: introduce `kernel::block::mq` module

Add initial abstractions for working with blk-mq.

This patch is a maintained, refactored subset of code originally published
by Wedson Almeida Filho <wedsonaf@gmail.com> [1].

[1] https://github.com/wedsonaf/linux/tree/f2cfd2fe0e2ca4e90994f96afe268bbd4382a891/rust/kernel/blk/mq.rs

Cc: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20240611114551.228679-2-nmi@metaspace.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 55 insertions, 0 deletions

diff --git a/rust/kernel/block/mq/raw_writer.rs b/rust/kernel/block/mq/raw_writer.rs
new file mode 100644
index 000000000000..9222465d670b
--- /dev/null
+++ b/rust/kernel/block/mq/raw_writer.rs
@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: GPL-2.0
+
+use core::fmt::{self, Write};
+
+use crate::error::Result;
+use crate::prelude::EINVAL;
+
+/// A mutable reference to a byte buffer where a string can be written into.
+///
+/// # Invariants
+///
+/// `buffer` is always null terminated.
+pub(crate) struct RawWriter<'a> {
+    buffer: &'a mut [u8],
+    pos: usize,
+}
+
+impl<'a> RawWriter<'a> {
+    /// Create a new `RawWriter` instance.
+    fn new(buffer: &'a mut [u8]) -> Result<RawWriter<'a>> {
+        *(buffer.last_mut().ok_or(EINVAL)?) = 0;
+
+        // INVARIANT: We null terminated the buffer above.
+        Ok(Self { buffer, pos: 0 })
+    }
+
+    pub(crate) fn from_array<const N: usize>(
+        a: &'a mut [core::ffi::c_char; N],
+    ) -> Result<RawWriter<'a>> {
+        Self::new(
+            // SAFETY: the buffer of `a` is valid for read and write as `u8` for
+            // at least `N` bytes.
+            unsafe { core::slice::from_raw_parts_mut(a.as_mut_ptr().cast::<u8>(), N) },
+        )
+    }
+}
+
+impl Write for RawWriter<'_> {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        let bytes = s.as_bytes();
+        let len = bytes.len();
+
+        // We do not want to overwrite our null terminator
+        if self.pos + len > self.buffer.len() - 1 {
+            return Err(fmt::Error);
+        }
+
+        // INVARIANT: We are not overwriting the last byte
+        self.buffer[self.pos..self.pos + len].copy_from_slice(bytes);
+
+        self.pos += len;
+
+        Ok(())
+    }
+}