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

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Message-Id: <DDZZX8AJKTP3.2OTD2L9ULVUG0@kernel.org>
Date: Tue, 04 Nov 2025 15:55:51 +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 Tue Nov 4, 2025 at 3:31 PM CET, Alice Ryhl wrote:
> On Tue, Nov 04, 2025 at 10:13:08AM +0100, Danilo Krummrich wrote:
>> 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.
>
> I guess it's more that you don't really need mutable access to call
> start_ptr_mut() for this particular case?

In general I think it's good to require a mutable reference for start_ptr_mut(),
as in the absolute majority of cases we actually want to inherit exclusive
access.

As you say, in this particular case we do not want to inherit exclusive access
on purpose.