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

[Matthew Auld <matthew.auld@intel.com>]

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 {})
> +    }
> +}