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

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Message-ID: <ad2963c2-3d44-4b6d-b975-75a9ea96ba1c@kernel.org>
Date: Tue, 4 Nov 2025 10:13:08 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
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 11/4/25 9:39 AM, Alice Ryhl wrote:
> 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?

item_from_index() is used for the dma_read!() and dma_write!() macros, hence
this one is on purpose.