linux-kernel - Re: [PATCH 1/2] rust: dma: make use of start_ptr() and start_ptr

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Message-ID: <aQm7qLybSn6bOlCy@google.com>
Date: Tue, 4 Nov 2025 08:39:04 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: abdiel.janulgue@...il.com, daniel.almeida@...labora.com, 
	robin.murphy@....com, a.hindborg@...nel.org, ojeda@...nel.org, 
	alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, 
	bjorn3_gh@...tonmail.com, lossin@...nel.org, tmgross@...ch.edu, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 1/2] rust: dma: make use of start_ptr() and start_ptr_mut()

On Mon, Nov 03, 2025 at 08:06:49PM +0100, Danilo Krummrich wrote:
> Using start_ptr() and start_ptr_mut() has the advantage that we inherit
> the requirements the a mutable or immutable reference from those
> methods.
> 
> Hence, use them instead of self.cpu_addr.
> 
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>

>          // and we've just checked that the range and index is within bounds.
>          // - `offset` can't overflow since it is smaller than `self.count` and we've checked
>          // that `self.count` won't overflow early in the constructor.
> -        Ok(unsafe { self.cpu_addr.add(offset) })
> +        Ok(unsafe { self.start_ptr().cast_mut().add(offset) })

I guess this shows that the mutable/immutable requirements we inherit
aren't actually what we need?

Alice