Re: [PATCH v7 5/7] drm/tyr: add a kernel buffer object

[Daniel Almeida <daniel.almeida@collabora.com>]

> On 14 Jul 2026, at 00:18, Deborah Brouwer <deborah.brouwer@collabora.com> wrote:
> 
> 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>
> ---
> drivers/gpu/drm/tyr/gem.rs | 101 +++++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 97 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tyr/gem.rs b/drivers/gpu/drm/tyr/gem.rs
> index c28be61a01bb..47a05a33388e 100644
> --- a/drivers/gpu/drm/tyr/gem.rs
> +++ b/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 +67,85 @@ 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<'bound> {
> +    /// The underlying GEM buffer object.
> +    #[expect(dead_code)]
> +    pub(crate) bo: ARef<Bo>,

Can we make this private?

> +    /// The GPU VM this buffer is mapped into.
> +    vm: Arc<Vm<'bound>>,
> +    /// The GPU VA range occupied by this buffer.
> +    va_range: Range<u64>,
> +}
> +
> +impl<'bound> KernelBo<'bound> {
> +    /// 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<'bound>>,
> +        size: u64,
> +        va_alloc: KernelBoVaAlloc,
> +        flags: VmMapFlags,
> +    ) -> Result<Self> {
> +        if size == 0 {
> +            pr_err!("Cannot create KernelBo with size 0\n");
> +            return Err(EINVAL);
> +        }
> +
> +        let KernelBoVaAlloc::Explicit(va) = va_alloc;
> +
> +        let bo = Bo::new(
> +            ddev,
> +            size as usize,

As a general rule, let’s try our best to reduce the number of `as` casts. Let’s go with
usize::try_from

> +            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 + size),
> +        })
> +    }
> +}
> +
> +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) {
> +            pr_err!(
> +                "Failed to unmap KernelBo range {:#x}..{:#x}: {:?}\n",
> +                self.va_range.start,
> +                self.va_range.end,
> +                e
> +            );
> +        }
> +    }
> +}
> 
> -- 
> 2.54.0
>