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Date: Wed, 25 Mar 2026 18:18:08 +0100
Message-Id: <DHC18ZPBO3X4.1P6GJKSE0IMK8@kernel.org>
Subject: Re: [PATCH 2/2] rust: dma: add CoherentHandle for DMA allocations
 without kernel mapping
Cc: <abdiel.janulgue@gmail.com>, <daniel.almeida@collabora.com>,
 <robin.murphy@arm.com>, <a.hindborg@kernel.org>, <ojeda@kernel.org>,
 <boqun@kernel.org>, <bjorn3_gh@protonmail.com>, <lossin@kernel.org>,
 <aliceryhl@google.com>, <tmgross@umich.edu>, <driver-core@lists.linux.dev>,
 <rust-for-linux@vger.kernel.org>, <linux-kernel@vger.kernel.org>
To: "Gary Guo" <gary@garyguo.net>
From: "Danilo Krummrich" <dakr@kernel.org>
References: <20260321172749.592387-1-dakr@kernel.org>
 <20260321172749.592387-2-dakr@kernel.org>
 <DHBAADML4BXP.131CD8C26BELH@garyguo.net>
 <DHBD2BO4R5BM.3OS7AHYN9A7DZ@kernel.org>
 <DHBVYVW7ZTDA.23N0Z3YOC5735@garyguo.net>
In-Reply-To: <DHBVYVW7ZTDA.23N0Z3YOC5735@garyguo.net>

On Wed Mar 25, 2026 at 2:09 PM CET, Gary Guo wrote:
> This only gives you fixed size, though. If you want the same type that su=
pports
> both a fixed size and dynamic size, then a generic that is either array/s=
lice is
> the way to go.

[...]

> You still can use `io_project!(handle, [start..end]?)` to do the bounds
> checking. But as you said, the benefit might be minimal.

I think we shouldn't overengineer this for now, and wait for actual use-cas=
es
that require any of that.

The only thing we need right now in practice is an API that takes the size =
and
returns a handle to the buffer of this size exposing the DMA address that i=
s
subsequently passed to the device without further modification.

So, if we want to re-use dma::Coherent<T>, we can just do this

	pub struct CoherentHandle(Coherent<[u8]>);
=09
	impl CoherentHandle {
	    pub fn alloc_with_attrs(
	        dev: &device::Device<Bound>,
	        size: usize,
	        gfp_flags: kernel::alloc::Flags,
	        dma_attrs: Attrs,
	    ) -> Result<Self> {
	        let dma_attrs =3D dma_attrs | Attrs(bindings::DMA_ATTR_NO_KERNEL_M=
APPING);
	        Coherent::<u8>::alloc_slice_with_attrs(dev, size, gfp_flags, dma_a=
ttrs).map(Self)
	    }
=09
	    #[inline]
	    pub fn alloc(
	        dev: &device::Device<Bound>,
	        size: usize,
	        gfp_flags: kernel::alloc::Flags,
	    ) -> Result<Self> {
	        Self::alloc_with_attrs(dev, size, gfp_flags, Attrs(0))
	    }
=09
	    #[inline]
	    pub fn dma_handle(&self) -> DmaAddress {
	        self.0.dma_handle()
	    }
=09
	    #[inline]
	    pub fn size(&self) -> usize {
	        self.0.size()
	    }
	}

and add a guarantee that Coherent::alloc_with_attrs() and Coherent::drop() =
never
touch the cpu_ptr.