linux-kernel - Re: [PATCH RFC 3/4] rust: drm: Add DRM buddy allocator bindings

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Message-Id: <DDX3S62U4O46.133LFLRFHMZI3@nvidia.com>
Date: Sat, 01 Nov 2025 14:19:18 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Joel Fernandes" <joelagnelf@...dia.com>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <dri-devel@...ts.freedesktop.org>, <dakr@...nel.org>, "David Airlie"
 <airlied@...il.com>
Cc: <acourbot@...dia.com>, "Alistair Popple" <apopple@...dia.com>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun
 Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Simona Vetter" <simona@...ll.ch>,
 "Maarten Lankhorst" <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard"
 <mripard@...nel.org>, "Thomas Zimmermann" <tzimmermann@...e.de>, "John
 Hubbard" <jhubbard@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
 <joel@...lfernandes.org>, "Elle Rhumsaa" <elle@...thered-steel.dev>,
 "Daniel Almeida" <daniel.almeida@...labora.com>, "Andrea Righi"
 <arighi@...dia.com>, "Philipp Stanner" <phasta@...nel.org>,
 <nouveau@...ts.freedesktop.org>, "Nouveau"
 <nouveau-bounces@...ts.freedesktop.org>
Subject: Re: [PATCH RFC 3/4] rust: drm: Add DRM buddy allocator bindings

On Fri Oct 31, 2025 at 4:06 AM JST, Joel Fernandes wrote:
<snip>
> +    /// Allocate blocks from the buddy allocator.
> +    ///
> +    /// Returns an [`AllocatedBlocks`] structure that owns the allocated blocks and automatically
> +    /// frees them when dropped. Allocation of `list_head` uses the `gfp` flags passed.
> +    pub fn alloc_blocks(
> +        &self,
> +        start: usize,
> +        end: usize,
> +        size: usize,
> +        min_block_size: usize,
> +        flags: BuddyFlags,
> +        gfp: Flags,
> +    ) -> Result<AllocatedBlocks<'_>> {

It looks like some of the flags heavily modify the behavior of the
allocator, and make some of the parameters irrelevant (for instance,
IIUC `start` and `end` only make sense if `RANGE_ALLOCATION` is
passed?).

In that case, we should either have several allocation methods, or have
an `AllocationRequest` enum which variants are the allocation type and its
relevant parameters. E.g (very naively):

enum AllocationRange {
    Whole,
    Ranged(Range<u64>),
    ...
}

struct AllocationRequest {
    range: AllocationRange,
    top_down: bool,
    contiguous: bool,
    clear: bool,
}

I.e. use the type system to make sure we can only express things that
make sense, and never pass data that will end up being ignored.

If we do that well, I think we can drop the `BuddyFlags` type
altogether, which is great as it seems to serve several different
purposes.