Re: [PATCH v2] rust: devres: optimize type name allocation and fix truncation

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, May 26, 2026 at 05:28:25PM +0530, Aary Milind Kinge wrote:
> The unconditional 128-byte const array allocation for every unique
> `Devres<T>` caused unnecessary .rodata bloat in production builds,
> and type names exceeding 127 bytes were silently truncated without
> any indication in debug logs.
> 
> Gate the `TYPE_NAME`, `TYPE_NAME_BUF`, and `TYPE_NAME_CSTR` constants
> behind `#[cfg(CONFIG_DEBUG_DEVRES)]` so the 128-byte buffer is only
> allocated when device resource debugging is enabled. For production
> builds, use `crate::c_str!("")` as a zero-cost empty C-string fallback.
> 
> When the type name is too long to fit in the 127-byte buffer (plus null
> terminator), the copy routine now appends "..." at the end — copying
> only the first 124 bytes — to clearly indicate truncation rather than
> silently dropping trailing characters.
> 
> Signed-off-by: Aary Milind Kinge <kingeaary@gmail.com>
> ---
>  rust/kernel/devres.rs | 55 ++++++++++++++++++++++++++++++++++++++++---
>  rust/kernel/lib.rs    |  1 +
>  2 files changed, 53 insertions(+), 3 deletions(-)
> 
> diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> index 9e5f93aed20c..79cfe7d9a415 100644
> --- a/rust/kernel/devres.rs
> +++ b/rust/kernel/devres.rs
> @@ -185,6 +185,57 @@ pub(super) unsafe fn devres_node_remove(
>  }
>  
>  impl<T: Send> Devres<T> {
> +    #[cfg(CONFIG_DEBUG_DEVRES)]
> +    const TYPE_NAME: &'static str = core::any::type_name::<T>();
> +
> +    #[cfg(CONFIG_DEBUG_DEVRES)]
> +    const TYPE_NAME_BUF: [u8; 128] = {
> +        let bytes = Self::TYPE_NAME.as_bytes();
> +        let mut buf = [0u8; 128];
> +        let mut i = 0;
> +
> +        if bytes.len() > 127 {
> +            // Copy exactly 124 bytes, then append '...' to clearly indicate truncation
> +            while i < 124 {
> +                buf[i] = bytes[i];
> +                i += 1;
> +            }
> +            buf[124] = b'.';
> +            buf[125] = b'.';
> +            buf[126] = b'.';
> +            buf[127] = 0; // Null terminator
> +        } else {
> +            // Copy normally
> +            while i < bytes.len() {
> +                buf[i] = bytes[i];
> +                i += 1;
> +            }
> +            buf[i] = 0; // Null terminator
> +        }
> +        buf
> +    };
> +
> +    #[cfg(CONFIG_DEBUG_DEVRES)]
> +    const TYPE_NAME_CSTR: &'static crate::str::CStr = {
> +        let static_buf: &'static [u8; 128] = &Self::TYPE_NAME_BUF;
> +
> +        let mut len = 0;
> +        while len < 128 && static_buf[len] != 0 {
> +            len += 1;
> +        }
> +
> +        // SAFETY: `static_buf` is promoted to static memory, and we verified the null byte.
> +        unsafe {
> +            crate::str::CStr::from_bytes_with_nul_unchecked(core::slice::from_raw_parts(
> +                static_buf.as_ptr(),
> +                len + 1,
> +            ))
> +        }
> +    };
> +
> +    #[cfg(not(CONFIG_DEBUG_DEVRES))]
> +    const TYPE_NAME_CSTR: &'static crate::str::CStr = crate::c_str!("");
> +
>      /// Creates a new [`Devres`] instance of the given `data`.
>      ///
>      /// The `data` encapsulated within the returned `Devres` instance' `data` will be
> @@ -209,9 +260,7 @@ pub fn new<E>(dev: &Device<Bound>, data: impl PinInit<T, E>) -> Result<Self>
>                      unsafe {
>                          base::devres_set_node_dbginfo(
>                              node,
> -                            // TODO: Use `core::any::type_name::<T>()` once it is a `const fn`,
> -                            // such that we can convert the `&str` to a `&CStr` at compile-time.
> -                            c"Devres<T>".as_char_ptr(),
> +                            Self::TYPE_NAME_CSTR.as_char_ptr(),
>                              core::mem::size_of::<Revocable<T>>(),
>                          )
>                      };
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index b72b2fbe046d..48df181788f6 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -22,6 +22,7 @@
>  // Expected to become stable.
>  #![feature(arbitrary_self_types)]
>  #![feature(derive_coerce_pointee)]
> +#![feature(const_type_name)]
>  //
>  // To be determined.
>  #![feature(used_with_arg)]
> -- 
> 2.51.0
> 
> 

Hi,

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