linux-kernel - Re: [PATCH RFC] soc: qcom: socinfo: Re-implement in Rust

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Message-ID: <20251215121145.19b99bb0.gary@garyguo.net>
Date: Mon, 15 Dec 2025 12:11:45 +0000
From: Gary Guo <gary@...yguo.net>
To: Matthew Maurer <mmaurer@...gle.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>, Bjorn Andersson
 <andersson@...nel.org>, Konrad Dybcio <konradybcio@...nel.org>, Satya Durga
 Srinivasu Prabhala <satyap@...cinc.com>, Miguel Ojeda <ojeda@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>, Björn Roy Baron
 <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>, Danilo Krummrich <dakr@...nel.org>, Trilok Soni
 <tsoni@...cinc.com>, linux-kernel@...r.kernel.org,
 linux-arm-msm@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH RFC] soc: qcom: socinfo: Re-implement in Rust

On Sat, 13 Dec 2025 08:58:56 -0800
Matthew Maurer <mmaurer@...gle.com> wrote:

> Some options:
> 1. Make holes explicit
> ```
> pub(crate) const PMIC_MODELS: &[Option<&'str>] = &[
>   Some("foo"),
>   Some("bar"),
>   None,
>   Some("baz"),
>   // ...
> };
> ```
> This is the one I'd suggest if we want to get rid of the 92. It has
> the downside of some extra explicit `None` entries, but the array
> isn't *that* sparse.
> 
> 2. Factor out 92 into a constant.
> 3. Define the constant in terms of index/value pairs instead. I could
> use `const`-time code to produce the array we want:
> ```
> const PMIC_ENTRIES: &[(usize, &str)] = &[(1, "foo"), (9, "bar"), (42, "baz")];
> 
> const PMIC_MODELS_LEN: usize = {
>     let mut max = 0;
>     let mut i = 0;
>     while i < PMIC_ENTRIES.len() {
>         if PMIC_ENTRIES[i].0 > max {
>             max = PMIC_ENTRIES[i].0;
>         }
>         i += 1;
>     }
>     max + 1
> };
> 
> pub const PMIC_MODELS: [Option<&'static str>; PMIC_MODELS_LEN] = {
>     let mut models = [None; PMIC_MODELS_LEN];
>     let mut i = 0;
>     while i < PMIC_ENTRIES.len() {
>         let (idx, val) = PMIC_ENTRIES[i];
>         models[idx] = Some(val);
>         i += 1;
>     }
>     models
> };
> ```
> (The slightly icky looking loops are because not all features are
> available in const mode.)
> This seems a bit overkill for what's going on.

How about making a macro for this and make it available from kernel crate?
Looks like this should do what you need?


/// Create a sparse array of `[Option<T>; _]`.
macro_rules! sparse_array {
    ($(
        $index:literal: $value:expr
    ),* $(,)?) => {{
        const SIZE: usize = {
            let mut size = 0;
            $(if $index >= size {
                size = $index + 1;
            })*
            size
        };

        const {
            let mut arr = [None; SIZE];
            $(arr[$index] = Some($value);)*
            arr
        }
    }}
}

fn main() {
    const EXAMPLE: &[Option<u32>] = &sparse_array! {
        0: 1,
        5: 2,
    };
    println!("{:?}", EXAMPLE);
}

Best,
Gary