Re: [PATCH RFC 4/4] samples: rust: Add sample demonstrating DRM buddy allocator

Matthew Auld
2025-10-31T16:42:33Z

On 30/10/2025 19:06, Joel Fernandes wrote:
> Demonstrates usage of the DRM buddy allocator bindings through
> a simple test module that initializes the allocator, performs
> allocations, and prints information about the allocated blocks.
>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
> samples/rust/Kconfig | 14 +++++
> samples/rust/Makefile | 1 +
> samples/rust/rust_drm_buddy.rs | 106 +++++++++++++++++++++++++++++++++
> 3 files changed, 121 insertions(+)
> create mode 100644 samples/rust/rust_drm_buddy.rs
>
> diff --git a/samples/rust/Kconfig b/samples/rust/Kconfig
> index b45631e2593c..8ccb4064ba91 100644
> --- a/samples/rust/Kconfig
> +++ b/samples/rust/Kconfig
> @@ -21,6 +21,20 @@ config SAMPLE_RUST_CLIST
>
> If unsure, say N.
>
> +config SAMPLE_RUST_DRM_BUDDY
> + tristate "DRM buddy allocator sample"
> + depends on DRM_BUDDY
> + help
> + This option builds the Rust DRM buddy allocator sample.
> +
> + The sample demonstrates using the DRM buddy allocator bindings
> + to allocate and free memory blocks.
> +
> + To compile this as a module, choose M here:
> + the module will be called rust_drm_buddy.
> +
> + If unsure, say N.
> +
> config SAMPLE_RUST_CONFIGFS
> tristate "Configfs sample"
> depends on CONFIGFS_FS
> diff --git a/samples/rust/Makefile b/samples/rust/Makefile
> index f8899c0df762..a56204ee4e96 100644
> --- a/samples/rust/Makefile
> +++ b/samples/rust/Makefile
> @@ -2,6 +2,7 @@
> ccflags-y += -I$(src) # needed for trace events
>
> obj-$(CONFIG_SAMPLE_RUST_CLIST) += rust_clist.o
> +obj-$(CONFIG_SAMPLE_RUST_DRM_BUDDY) += rust_drm_buddy.o
> obj-$(CONFIG_SAMPLE_RUST_MINIMAL) += rust_minimal.o
> obj-$(CONFIG_SAMPLE_RUST_MISC_DEVICE) += rust_misc_device.o
> obj-$(CONFIG_SAMPLE_RUST_PRINT) += rust_print.o
> diff --git a/samples/rust/rust_drm_buddy.rs b/samples/rust/rust_drm_buddy.rs
> new file mode 100644
> index 000000000000..96907bc19243
> --- /dev/null
> +++ b/samples/rust/rust_drm_buddy.rs
> @@ -0,0 +1,106 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Rust DRM buddy allocator sample.
> +//!
> +//! This sample demonstrates using the DRM buddy allocator from Rust.
> +
> +use kernel::{
> + drm::buddy::{
> + BuddyFlags,
> + DrmBuddy, //
> + },
> + prelude::*,
> + sizes::*, //
> +};
> +
> +module! {
> + type: RustDrmBuddySample,
> + name: "rust_drm_buddy",
> + authors: ["Joel Fernandes"],
> + description: "DRM buddy allocator sample",
> + license: "GPL",
> +}
> +
> +struct RustDrmBuddySample;
> +
> +impl kernel::Module for RustDrmBuddySample {
> + fn init(_module: &'static ThisModule) -> Result<Self> {
> + // Create a buddy allocator managing 1GB with 4KB chunks.
> + let buddy = DrmBuddy::new(SZ_1G, SZ_4K)?;
> +
> + pr_info!("=== Test 1: Single 16MB block ===\n");
> + let allocated = buddy.alloc_blocks(
> + 0,
> + 0,

Does this map to the start/end? Surprised that this works with
RANGE_ALLOCATION below. I guess it works because of the end-1, but I'm
not sure if that was intended.

Anyway, probably you didn't really want RANGE_ALLOCATION here? That is
only if you want something at a specific offset or within a special bias
range. So here I think it will give you a massive bias range covering
everything due to end-1, but all you wanted was any available 16M block,
which is the typical flow? It still technically works, but looks a bit
non-standard and will internally take the bias range path, which is not
ideal :)

Also I guess worth updating the example in buddy.rs, which also does this?

> + SZ_16M,
> + SZ_4K,
> + BuddyFlags::RANGE_ALLOCATION,
> + GFP_KERNEL,
> + )?;
> +
> + let mut count = 0;
> + for block in &allocated {
> + pr_info!(
> + " Block {}: offset=0x{:x}, order={}, size={}\n",
> + count,
> + block.offset(),
> + block.order(),
> + block.size(&buddy)
> + );
> + count += 1;
> + }
> + pr_info!(" Total: {} blocks\n", count);
> + drop(allocated);
> +
> + pr_info!("=== Test 2: Three 4MB blocks ===\n");
> + let allocated = buddy.alloc_blocks(
> + 0,
> + 0,
> + SZ_4M * 3,
> + SZ_4K,
> + BuddyFlags::RANGE_ALLOCATION,
> + GFP_KERNEL,
> + )?;
> +
> + count = 0;
> + for block in &allocated {
> + pr_info!(
> + " Block {}: offset=0x{:x}, order={}, size={}\n",
> + count,
> + block.offset(),
> + block.order(),
> + block.size(&buddy)
> + );
> + count += 1;
> + }
> + pr_info!(" Total: {} blocks\n", count);
> + drop(allocated);
> +
> + pr_info!("=== Test 3: Two 8MB blocks ===\n");
> + let allocated = buddy.alloc_blocks(
> + 0,
> + 0,
> + SZ_8M * 2,
> + SZ_4K,
> + BuddyFlags::RANGE_ALLOCATION,
> + GFP_KERNEL,
> + )?;
> +
> + count = 0;
> + for block in &allocated {
> + pr_info!(
> + " Block {}: offset=0x{:x}, order={}, size={}\n",
> + count,
> + block.offset(),
> + block.order(),
> + block.size(&buddy)
> + );
> + count += 1;
> + }
> + pr_info!(" Total: {} blocks\n", count);
> +
> + pr_info!("=== All tests passed! ===\n");
> +
> + Ok(RustDrmBuddySample {})
> + }
> +}