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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774360848; bh=c9NFAllBfGAlx/FFc3N9mnr4Kiv07CkCPahF9j8WHzw=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=OlxVR12oL/BTtAOUzCP9vu1WlUnqcDXriXSDyGTfV4n87zTnVvAExFtGIIvowDd/m zOEW5egUKzOahJ7/5wCpP6j88bE6760LMadtlzD/d8hnjU25KldG9nlYM+xO+a3Ykf ywyzVhtyJWNTlEE/uLCnC2XFN9umt5d356KuUfXl5jrXjRg9uB9w9s8njzpcC7l3zg z5Cy9vb4FutpeMYS65wWUriqKLaEYgh36dPFB+NYlvLXcwwoVhTiuhSF6sDCrVK1Vh 9MwnAOMySimEIbS6CayPYAljjpevcCZVq6lVuoF3+JW2D3N1jARYzm50iJQ+qyTZrD 1P3mmQylNIUUA== From: Andreas Hindborg To: Danilo Krummrich , aliceryhl@google.com, acourbot@nvidia.com, ojeda@kernel.org, boqun@kernel.org, gary@garyguo.net, bjorn3_gh@protonmail.com, lossin@kernel.org, tmgross@umich.edu, abdiel.janulgue@gmail.com, daniel.almeida@collabora.com, robin.murphy@arm.com Subject: Re: [PATCH v2 4/8] rust: dma: introduce dma::CoherentBox for memory initialization In-Reply-To: <20260320194626.36263-5-dakr@kernel.org> References: <20260320194626.36263-1-dakr@kernel.org> <20260320194626.36263-5-dakr@kernel.org> Date: Tue, 24 Mar 2026 14:57:56 +0100 Message-ID: <871ph9qqhn.fsf@kernel.org> MIME-Version: 1.0 Content-Type: text/plain Message-ID-Hash: YJYQXMORQA6XPORPRHF4IT6ZODKQ5NMQ X-Message-ID-Hash: YJYQXMORQA6XPORPRHF4IT6ZODKQ5NMQ X-MailFrom: a.hindborg@kernel.org X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: driver-core@lists.linux.dev, nouveau@lists.freedesktop.org, dri-devel@lists.freedesktop.org, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, Danilo Krummrich X-Mailman-Version: 3.3.8 Precedence: list List-Id: Nouveau development list Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: "Danilo Krummrich" writes: > Currently, dma::Coherent cannot safely provide (mutable) access to its > underlying memory because the memory might be concurrently accessed by a > DMA device. This makes it difficult to safely initialize the memory > before handing it over to the hardware. > > Introduce dma::CoherentBox, a type that encapsulates a dma::Coherent > before its DMA address is exposed to the device. dma::CoherentBox can > guarantee exclusive access to the inner dma::Coherent and implement > Deref and DerefMut. > > Once the memory is properly initialized, dma::CoherentBox can be > converted into a regular dma::Coherent. > > Reviewed-by: Alice Ryhl > Signed-off-by: Danilo Krummrich > --- > rust/kernel/dma.rs | 154 ++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 153 insertions(+), 1 deletion(-) > > diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs > index db645b01bdd0..cefb54f0424a 100644 > --- a/rust/kernel/dma.rs > +++ b/rust/kernel/dma.rs > @@ -20,7 +20,13 @@ > FromBytes, // > }, // > }; > -use core::ptr::NonNull; > +use core::{ > + ops::{ > + Deref, > + DerefMut, // > + }, > + ptr::NonNull, // > +}; > > /// DMA address type. > /// > @@ -352,6 +358,152 @@ fn from(direction: DataDirection) -> Self { > } > } > > +/// CPU-owned DMA allocation that can be converted into a device-shared [`Coherent`] object. > +/// > +/// Unlike [`Coherent`], a [`CoherentBox`] is guaranteed to be fully owned by the CPU -- its DMA > +/// address is not exposed and it cannot be accessed by a device. This means it can safely be used > +/// like a normal boxed allocation (e.g. direct reads, writes, and mutable slices are all safe). > +/// > +/// A typical use is to allocate a [`CoherentBox`], populate it with normal CPU access, and then > +/// convert it into a [`Coherent`] object to share it with the device. > +/// > +/// # Examples > +/// > +/// `CoherentBox`: > +/// > +/// ``` > +/// # use kernel::device::{ > +/// # Bound, > +/// # Device, > +/// # }; > +/// use kernel::dma::{attrs::*, > +/// Coherent, > +/// CoherentBox, > +/// }; > +/// > +/// # fn test(dev: &Device) -> Result { > +/// let mut dmem: CoherentBox = CoherentBox::zeroed(dev, GFP_KERNEL)?; > +/// *dmem = 42; > +/// let dmem: Coherent = dmem.into(); > +/// # Ok::<(), Error>(()) } > +/// ``` > +/// > +/// `CoherentBox<[T]>`: > +/// > +/// > +/// ``` > +/// # use kernel::device::{ > +/// # Bound, > +/// # Device, > +/// # }; > +/// use kernel::dma::{attrs::*, > +/// Coherent, > +/// CoherentBox, > +/// }; > +/// > +/// # fn test(dev: &Device) -> Result { > +/// let mut dmem: CoherentBox<[u64]> = CoherentBox::zeroed_slice(dev, 4, GFP_KERNEL)?; > +/// dmem.fill(42); > +/// let dmem: Coherent<[u64]> = dmem.into(); > +/// # Ok::<(), Error>(()) } > +/// ``` > +pub struct CoherentBox(Coherent); > + > +impl CoherentBox<[T]> { > + /// [`CoherentBox`] variant of [`Coherent::zeroed_slice_with_attrs`]. > + #[inline] > + pub fn zeroed_slice_with_attrs( > + dev: &device::Device, > + count: usize, > + gfp_flags: kernel::alloc::Flags, > + dma_attrs: Attrs, > + ) -> Result { > + Coherent::zeroed_slice_with_attrs(dev, count, gfp_flags, dma_attrs).map(Self) > + } > + > + /// Same as [CoherentBox::zeroed_slice_with_attrs], but with `dma::Attrs(0)`. > + #[inline] > + pub fn zeroed_slice( > + dev: &device::Device, > + count: usize, > + gfp_flags: kernel::alloc::Flags, > + ) -> Result { > + Self::zeroed_slice_with_attrs(dev, count, gfp_flags, Attrs(0)) > + } > + > + /// Initializes the element at `i` using the given initializer. > + /// > + /// Returns `EINVAL` if `i` is out of bounds. Could you add an example for this function? Reviewed-by: Andreas Hindborg Best regards, Andreas Hindborg