[RFC PATCH 1/2] rust: block: mq: safely expose TagSet flags

[Wenzhao Liao]

TagSet::new() currently hardcodes blk_mq_tag_set.flags to 0.

That prevents Rust block drivers from declaring blk-mq queue flags.

Introduce TagSetFlags as a typed wrapper for BLK_MQ_F_* bits.

Add TagSet::new_with_flags() so drivers can pass flags explicitly.

Keep TagSet::new() as a compatibility wrapper using empty flags.

Re-export TagSetFlags from kernel::block::mq for driver imports.

Build-tested with LLVM=-15 in an out-of-tree rust-next build.

Validation includes vmlinux and drivers/block/rnull/rnull_mod.ko.

Signed-off-by: Wenzhao Liao <wenzhaoliao@xxxxxxxxxx>
---
 rust/kernel/block/mq.rs         |  2 +-
 rust/kernel/block/mq/tag_set.rs | 86 ++++++++++++++++++++++++++++++++-
 2 files changed, 86 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/block/mq.rs b/rust/kernel/block/mq.rs
index 1fd0d54dd549..799afdf36539 100644
--- a/rust/kernel/block/mq.rs
+++ b/rust/kernel/block/mq.rs
@@ -100,4 +100,4 @@
 
 pub use operations::Operations;
 pub use request::Request;
-pub use tag_set::TagSet;
+pub use tag_set::{TagSet, TagSetFlags};
diff --git a/rust/kernel/block/mq/tag_set.rs b/rust/kernel/block/mq/tag_set.rs
index dae9df408a86..72d9bce5b11f 100644
--- a/rust/kernel/block/mq/tag_set.rs
+++ b/rust/kernel/block/mq/tag_set.rs
@@ -16,6 +16,80 @@
 use core::{convert::TryInto, marker::PhantomData};
 use pin_init::{pin_data, pinned_drop, PinInit};
 
+/// Flags that control blk-mq tag set behavior.
+///
+/// They can be combined with the operators `|`, `&`, and `!`.
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub struct TagSetFlags(u32);
+
+impl TagSetFlags {
+    /// Returns an empty instance where no flags are set.
+    pub const fn empty() -> Self {
+        Self(0)
+    }
+
+    /// Register as a blocking blk-mq driver device.
+    pub const BLOCKING: Self = Self::new(bindings::BLK_MQ_F_BLOCKING as u32);
+
+    /// Use an underlying blk-mq device for completing I/O.
+    pub const STACKING: Self = Self::new(bindings::BLK_MQ_F_STACKING as u32);
+
+    /// Share hardware contexts between tags.
+    pub const TAG_HCTX_SHARED: Self = Self::new(bindings::BLK_MQ_F_TAG_HCTX_SHARED as u32);
+
+    /// Allocate tags on a round-robin basis.
+    pub const TAG_RR: Self = Self::new(bindings::BLK_MQ_F_TAG_RR as u32);
+
+    /// Disable the I/O scheduler by default.
+    pub const NO_SCHED_BY_DEFAULT: Self =
+        Self::new(bindings::BLK_MQ_F_NO_SCHED_BY_DEFAULT as u32);
+
+    /// Check whether `flags` is contained in `self`.
+    pub fn contains(self, flags: Self) -> bool {
+        (self & flags) == flags
+    }
+
+    pub(crate) const fn as_raw(self) -> u32 {
+        self.0
+    }
+
+    const fn all_bits() -> u32 {
+        Self::BLOCKING.0
+            | Self::STACKING.0
+            | Self::TAG_HCTX_SHARED.0
+            | Self::TAG_RR.0
+            | Self::NO_SCHED_BY_DEFAULT.0
+    }
+
+    const fn new(value: u32) -> Self {
+        Self(value)
+    }
+}
+
+impl core::ops::BitOr for TagSetFlags {
+    type Output = Self;
+
+    fn bitor(self, rhs: Self) -> Self::Output {
+        Self(self.0 | rhs.0)
+    }
+}
+
+impl core::ops::BitAnd for TagSetFlags {
+    type Output = Self;
+
+    fn bitand(self, rhs: Self) -> Self::Output {
+        Self(self.0 & rhs.0)
+    }
+}
+
+impl core::ops::Not for TagSetFlags {
+    type Output = Self;
+
+    fn not(self) -> Self::Output {
+        Self(!self.0 & Self::all_bits())
+    }
+}
+
 /// A wrapper for the C `struct blk_mq_tag_set`.
 ///
 /// `struct blk_mq_tag_set` contains a `struct list_head` and so must be pinned.
@@ -37,6 +111,16 @@ pub fn new(
         nr_hw_queues: u32,
         num_tags: u32,
         num_maps: u32,
+    ) -> impl PinInit<Self, error::Error> {
+        Self::new_with_flags(nr_hw_queues, num_tags, num_maps, TagSetFlags::empty())
+    }
+
+    /// Try to create a new tag set with the given blk-mq flags.
+    pub fn new_with_flags(
+        nr_hw_queues: u32,
+        num_tags: u32,
+        num_maps: u32,
+        flags: TagSetFlags,
     ) -> impl PinInit<Self, error::Error> {
         let tag_set: bindings::blk_mq_tag_set = pin_init::zeroed();
         let tag_set: Result<_> = core::mem::size_of::<RequestDataWrapper>()
@@ -49,7 +133,7 @@ pub fn new(
                     numa_node: bindings::NUMA_NO_NODE,
                     queue_depth: num_tags,
                     cmd_size,
-                    flags: 0,
+                    flags: flags.as_raw(),
                     driver_data: core::ptr::null_mut::<crate::ffi::c_void>(),
                     nr_maps: num_maps,
                     ..tag_set
-- 
2.34.1