[PATCH v3 4/4] gpu: nova-core: add ChannelIdPool

Add `ChannelIdPool` which adds automatic tracking and releasing of
channel IDs on top of `IdPool`. This is necessary for apportioning
ranges of channel IDs to be used in e.g. vGPU.

Channel IDs are allocated as a contiguous sequence with a specific
length and sometimes a specific alignment [1] for vGPU. The ID space is
small (limited to 2048) and allocation is not on a hot path, so a
bitmap-backed `IdPool` is a better fit than IDA/xarray (which allocate a
single ID within a range, not a contiguous sequence) or a maple tree
(where aligned allocation needs an alloc_range()+erase() retry loop that
essentially reimplements bitmap_find_next_zero_area()) [2]. It is
also faster than maple tree [3].

Link: https://lore.kernel.org/all/[email protected]/ # [1]
Link: https://lore.kernel.org/all/[email protected]/ # [2]
Link: https://lore.kernel.org/all/[email protected]/ # [3]
Signed-off-by: Eliot Courtney <[email protected]>
---
 drivers/gpu/nova-core/gpu.rs         |   2 +
 drivers/gpu/nova-core/gpu/channel.rs | 180 +++++++++++++++++++++++++++++++++++
 2 files changed, 182 insertions(+)

diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
index 42a4cd7971fa..66ea697a89f8 100644
--- a/drivers/gpu/nova-core/gpu.rs
+++ b/drivers/gpu/nova-core/gpu.rs
@@ -33,6 +33,8 @@
     vgpu::VgpuManager, //
 };
 
+#[cfg_attr(not(CONFIG_KUNIT = "y"), expect(dead_code))]
+mod channel;
 mod hal;
 
 macro_rules! define_chipset {
diff --git a/drivers/gpu/nova-core/gpu/channel.rs b/drivers/gpu/nova-core/gpu/channel.rs
new file mode 100644
index 000000000000..b755d2184aee
--- /dev/null
+++ b/drivers/gpu/nova-core/gpu/channel.rs
@@ -0,0 +1,180 @@
+// SPDX-License-Identifier: GPL-2.0
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+
+//! Channel ID allocation.
+
+use core::{
+    num::NonZero,
+    ops::{
+        Deref,
+        Range, //
+    }, //
+};
+
+use kernel::{
+    id_pool::IdPool,
+    prelude::*,
+    ptr::Alignment,
+    sync::{
+        new_mutex,
+        Mutex, //
+    }, //
+};
+
+/// Pool for tracking reservations of channel IDs.
+#[pin_data]
+pub(crate) struct ChannelIdPool {
+    #[pin]
+    inner: Mutex<IdPool>,
+    num_chids: usize,
+}
+
+impl ChannelIdPool {
+    /// Creates a pool managing `num_chids` channel IDs.
+    pub(crate) fn new(num_chids: usize) -> impl PinInit<Self, Error> {
+        try_pin_init!(Self {
+            inner <- new_mutex!(IdPool::with_capacity(num_chids, GFP_KERNEL)?),
+            num_chids,
+        })
+    }
+
+    /// Reserves a contiguous area of `count` channel IDs starting at a multiple of `align`,
+    /// returning a guard that releases the area on drop.
+    pub(crate) fn alloc_area(
+        &self,
+        count: NonZero<usize>,
+        align: Alignment,
+    ) -> Result<ChannelIdArea<'_>> {
+        let mut ids = self.inner.lock();
+        let area = ids.find_unused_area(0, count, align).ok_or(ENOSPC)?;
+
+        // If the pool is small, the backing bitmap may be rounded up to a larger size.
+        if area.range().end > self.num_chids {
+            return Err(ENOSPC);
+        }
+        Ok(ChannelIdArea {
+            pool: self,
+            range: area.acquire(),
+        })
+    }
+}
+
+/// A reserved contiguous area of channel IDs.
+///
+/// Releases the whole area back to its [`ChannelIdPool`] when dropped. Releasing locks a
+/// sleeping [`Mutex`], so the area must be dropped in a context that is allowed to sleep.
+#[must_use = "the channel ID area is released immediately when unused"]
+pub(crate) struct ChannelIdArea<'a> {
+    pool: &'a ChannelIdPool,
+    range: Range<usize>,
+}
+
+impl Drop for ChannelIdArea<'_> {
+    fn drop(&mut self) {
+        self.pool.inner.lock().release_area(&self.range);
+    }
+}
+
+impl Deref for ChannelIdArea<'_> {
+    type Target = Range<usize>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.range
+    }
+}
+
+#[kunit_tests(nova_core_channel)]
+mod tests {
+    use super::*;
+
+    const fn nz<const N: usize>() -> NonZero<usize> {
+        const { NonZero::new(N).unwrap() }
+    }
+
+    #[test]
+    fn chid_area() -> Result {
+        let pool = KBox::pin_init(ChannelIdPool::new(2048), GFP_KERNEL)?;
+        let unaligned = Alignment::new::<1>();
+
+        let first = pool.alloc_area(nz::<48>(), unaligned)?;
+        assert_eq!(0, first.start);
+        assert_eq!(48, first.len());
+        assert_eq!(48, first.end);
+
+        let second = pool.alloc_area(nz::<48>(), unaligned)?;
+        assert!(first.end <= second.start || second.end <= first.start);
+
+        let first_start = first.start;
+        drop(first);
+        assert_eq!(first_start, pool.alloc_area(nz::<48>(), unaligned)?.start);
+        Ok(())
+    }
+
+    #[test]
+    fn chid_bounded_by_num_chids() -> Result {
+        let pool = KBox::pin_init(ChannelIdPool::new(4), GFP_KERNEL)?;
+        let unaligned = Alignment::new::<1>();
+
+        {
+            let a = pool.alloc_area(nz::<1>(), unaligned)?;
+            let b = pool.alloc_area(nz::<1>(), unaligned)?;
+            let c = pool.alloc_area(nz::<1>(), unaligned)?;
+            let d = pool.alloc_area(nz::<1>(), unaligned)?;
+            assert_eq!(0, a.start);
+            assert_eq!(1, b.start);
+            assert_eq!(2, c.start);
+            assert_eq!(3, d.start);
+            assert_eq!(
+                Err(ENOSPC),
+                pool.alloc_area(nz::<1>(), unaligned).map(|_| ())
+            );
+        }
+
+        assert_eq!(0, pool.alloc_area(nz::<4>(), unaligned)?.start);
+        assert_eq!(
+            Err(ENOSPC),
+            pool.alloc_area(nz::<5>(), unaligned).map(|_| ())
+        );
+
+        let head = pool.alloc_area(nz::<3>(), unaligned)?;
+        assert_eq!(0, head.start);
+        assert_eq!(
+            Err(ENOSPC),
+            pool.alloc_area(nz::<2>(), unaligned).map(|_| ())
+        );
+        assert_eq!(3, pool.alloc_area(nz::<1>(), unaligned)?.start);
+        Ok(())
+    }
+
+    #[test]
+    fn chid_area_aligned() -> Result {
+        let pool = KBox::pin_init(ChannelIdPool::new(16), GFP_KERNEL)?;
+        let unaligned = Alignment::new::<1>();
+        let align4 = Alignment::new::<4>();
+
+        // Alloc 0 so the first fit for the next area is unaligned.
+        let pad = pool.alloc_area(nz::<1>(), unaligned)?;
+        assert_eq!(0, pad.start);
+
+        let a = pool.alloc_area(nz::<4>(), align4)?;
+        assert_eq!(4, a.start);
+
+        // The area skipped over by the aligned allocation should still be available.
+        let b = pool.alloc_area(nz::<1>(), unaligned)?;
+        assert_eq!(1, b.start);
+
+        let c = pool.alloc_area(nz::<8>(), Alignment::new::<8>())?;
+        assert_eq!(8, c.start);
+
+        // Only 2 IDs left.
+        assert_eq!(Err(ENOSPC), pool.alloc_area(nz::<4>(), align4).map(|_| ()));
+        assert_eq!(
+            Err(ENOSPC),
+            pool.alloc_area(nz::<1>(), Alignment::new::<32>())
+                .map(|_| ())
+        );
+
+        assert_eq!(2, pool.alloc_area(nz::<2>(), unaligned)?.start);
+        Ok(())
+    }
+}

-- 
2.55.0