drm/tyr: add a kernel buffer object

Introduce a buffer object type (KernelBo) for internal driver allocations
that are managed by the kernel rather than userspace.

KernelBo wraps a GEM shmem object and automatically handles GPU virtual
address space mapping during creation and unmapping on drop. This provides
a safe and convenient way for the driver to both allocate and clean up
internal buffers for kernel-managed resources.

Co-developed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Link: https://patch.msgid.link/20260728-fw-boot-b4-v10-5-9187aefa3f2f@collabora.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

drivers/gpu/drm/tyr/gem.rs

@@ -4,18 +4,29 @@
 //! This module provides buffer object (BO) management functionality using
 //! DRM's GEM subsystem with shmem backing.
 
+use core::ops::Range;
+
 use kernel::{
     drm::gem::{
         self,
         shmem, //
     },
     prelude::*,
-    sync::aref::ARef, //
+    sync::{
+        aref::ARef,
+        Arc, //
+    }, //
 };
 
-use crate::driver::{
-    TyrDrmDevice,
-    TyrDrmDriver, //
+use crate::{
+    driver::{
+        TyrDrmDevice,
+        TyrDrmDriver, //
+    },
+    vm::{
+        Vm,
+        VmMapFlags, //
+    },
 };
 
 /// Tyr's DriverObject type for GEM objects.
@@ -56,3 +67,97 @@ pub(crate) fn new_dummy_object(ddev: &TyrDrmDevice) -> Result<ARef<Bo>> {
 
     Ok(bo)
 }
+
+/// Specifies how to choose a GPU virtual address for a [`KernelBo`].
+/// An automatic VA allocation strategy will be added in the future.
+pub(crate) enum KernelBoVaAlloc {
+    /// Explicit VA address specified by the caller.
+    #[expect(dead_code)]
+    Explicit(u64),
+}
+
+/// A kernel-owned buffer object with automatic GPU virtual address mapping.
+///
+/// This structure represents a buffer object that is created and managed entirely
+/// by the kernel driver, as opposed to userspace-created GEM objects. It combines
+/// a GEM object with automatic GPU virtual address (VA) space mapping and cleanup.
+///
+/// When dropped, the buffer is automatically unmapped from the GPU VA space.
+pub(crate) struct KernelBo<'drm> {
+    /// The underlying GEM buffer object.
+    bo: ARef<Bo>,
+    /// The GPU VM this buffer is mapped into.
+    vm: Arc<Vm<'drm>>,
+    /// The GPU VA range occupied by this buffer.
+    va_range: Range<u64>,
+}
+
+impl<'drm> KernelBo<'drm> {
+    /// Creates a new kernel-owned buffer object and maps it into GPU VA space.
+    ///
+    /// This function allocates a new shmem-backed GEM object and immediately maps
+    /// it into the specified GPU virtual memory space. The mapping is automatically
+    /// cleaned up when the [`KernelBo`] is dropped.
+    #[expect(dead_code)]
+    pub(crate) fn new(
+        ddev: &TyrDrmDevice,
+        vm: Arc<Vm<'drm>>,
+        size: u64,
+        va_alloc: KernelBoVaAlloc,
+        flags: VmMapFlags,
+    ) -> Result<Self> {
+        if size == 0 {
+            dev_err!(vm.dev(), "Cannot create KernelBo with size 0");
+            return Err(EINVAL);
+        }
+
+        let KernelBoVaAlloc::Explicit(va) = va_alloc;
+
+        let bo_size = usize::try_from(size).map_err(|_| EOVERFLOW)?;
+        let va_end = va.checked_add(size).ok_or(EINVAL)?;
+
+        let bo = Bo::new(
+            ddev,
+            bo_size,
+            shmem::ObjectConfig {
+                map_wc: true,
+                parent_resv_obj: None,
+            },
+            BoCreateArgs { flags: 0 },
+        )?;
+
+        vm.map_bo_range(&bo, 0, size, va, flags)?;
+
+        Ok(KernelBo {
+            bo,
+            vm,
+            va_range: va..va_end,
+        })
+    }
+
+    #[expect(dead_code)]
+    pub(crate) fn bo(&self) -> &Bo {
+        &self.bo
+    }
+}
+
+impl Drop for KernelBo<'_> {
+    fn drop(&mut self) {
+        let va = self.va_range.start;
+        let size = self.va_range.end - self.va_range.start;
+
+        if let Err(e) = self.vm.unmap_range(va, size) {
+            // If unmap_range fails, it is still safe to drop the
+            // KernelBo and its ARef to the GEM buffer object because
+            // GPUVM also holds a reference to the GEM buffer object.
+            // The physical pages won't be freed or reallocated.
+            dev_err!(
+                self.vm.dev(),
+                "Failed to unmap KernelBo range {:#x}..{:#x}: {:?}",
+                self.va_range.start,
+                self.va_range.end,
+                e
+            );
+        }
+    }
+}