[PATCH v2 11/32] gpu: nova-core: mm: add VramRegion

[Zhi Wang <zhiw@nvidia.com>]

BAR mappings do not necessarily cover an entire VRAM block. A region
layer lets a mapping select a byte range while keeping the underlying
allocation alive.

For vGPU, communication buffers occupy only part of the management heap
and need a BAR mapping of that range.

Add VramRegion to describe ranges within a shared VramBlock and support
bounds-checked subregions.

Cc: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Zhi Wang <zhiw@nvidia.com>
---
 drivers/gpu/nova-core/mm/vram.rs | 62 ++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/drivers/gpu/nova-core/mm/vram.rs b/drivers/gpu/nova-core/mm/vram.rs
index 743a2a163318..2c7183f08027 100644
--- a/drivers/gpu/nova-core/mm/vram.rs
+++ b/drivers/gpu/nova-core/mm/vram.rs
@@ -34,6 +34,68 @@ impl VramBlock {
     pub(crate) const fn address(&self) -> u64 {
         self.address
     }
+
+    /// Creates a byte range relative to this allocation, retaining its backing storage.
+    pub(crate) fn region(self: &Arc<Self>, range: Range<u64>) -> Result<VramRegion> {
+        VramRegion::new(self.clone(), range)
+    }
+}
+
+/// A byte range that keeps its VRAM allocation alive.
+#[derive(Clone)]
+pub(crate) struct VramRegion {
+    backing: Arc<VramBlock>,
+    address: u64,
+    size: u64,
+}
+
+impl VramRegion {
+    fn new(backing: Arc<VramBlock>, range: Range<u64>) -> Result<Self> {
+        let size = range
+            .end
+            .checked_sub(range.start)
+            .filter(|size| *size != 0)
+            .ok_or(EINVAL)?;
+        if range.end > backing.size {
+            return Err(EINVAL);
+        }
+        let address = backing.address.checked_add(range.start).ok_or(EOVERFLOW)?;
+        backing.address.checked_add(range.end).ok_or(EOVERFLOW)?;
+
+        Ok(Self {
+            backing,
+            address,
+            size,
+        })
+    }
+
+    pub(crate) const fn address(&self) -> u64 {
+        self.address
+    }
+
+    pub(crate) const fn size(&self) -> u64 {
+        self.size
+    }
+
+    /// Creates a nonempty subregion relative to this view's start.
+    pub(crate) fn subregion(&self, range: Range<u64>) -> Result<Self> {
+        let size = range
+            .end
+            .checked_sub(range.start)
+            .filter(|size| *size != 0)
+            .ok_or(EINVAL)?;
+        if range.end > self.size {
+            return Err(EINVAL);
+        }
+        let address = self.address.checked_add(range.start).ok_or(EOVERFLOW)?;
+        address.checked_add(size).ok_or(EOVERFLOW)?;
+
+        Ok(Self {
+            backing: self.backing.clone(),
+            address,
+            size,
+        })
+    }
 }
 
 impl GpuMm<'_> {