linux-kernel - Re: [PATCH v2 2/3] nova-core: Simplify `DmaObject::from

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Message-Id: <DDXC0YKKJJT4.18JQ1MJQ7I31Y@nvidia.com>
Date: Sat, 01 Nov 2025 20:46:56 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Daniel del Castillo" <delcastillodelarosadaniel@...il.com>, "Danilo
 Krummrich" <dakr@...nel.org>, "Alexandre Courbot" <acourbot@...dia.com>,
 "David Airlie" <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>,
 "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>
Cc: <nouveau@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
 <linux-kernel@...r.kernel.org>, "Boqun Feng" <boqun.feng@...il.com>, "Gary
 Guo" <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, <rust-for-linux@...r.kernel.org>,
 "dri-devel" <dri-devel-bounces@...ts.freedesktop.org>
Subject: Re: [PATCH v2 2/3] nova-core: Simplify `DmaObject::from_data` in
 nova-core/dma.rs

On Fri Oct 24, 2025 at 5:51 AM JST, Daniel del Castillo wrote:
> This patch solves one of the existing mentions of COHA, a task
> in the Nova task list about improving the `CoherentAllocation` API.
> It uses the `write` method from `CoherentAllocation`.
>
> Signed-off-by: Daniel del Castillo <delcastillodelarosadaniel@...il.com>
>
> ---
>
> V1 -> V2: Split previous patch into two. One per reference to COHA.
>           Added more details in Safety comment. Let me know your thoughts
>           Kept the original map to avoid a temporary variable
> ---
>  drivers/gpu/nova-core/dma.rs | 15 +++------------
>  1 file changed, 3 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/nova-core/dma.rs b/drivers/gpu/nova-core/dma.rs
> index 94f44bcfd748..620d31078858 100644
> --- a/drivers/gpu/nova-core/dma.rs
> +++ b/drivers/gpu/nova-core/dma.rs
> @@ -26,18 +26,9 @@ pub(crate) fn new(dev: &device::Device<device::Bound>, len: usize) -> Result<Sel
>  
>      pub(crate) fn from_data(dev: &device::Device<device::Bound>, data: &[u8]) -> Result<Self> {
>          Self::new(dev, data.len()).map(|mut dma_obj| {
> -            // TODO[COHA]: replace with `CoherentAllocation::write()` once available.
> -            // SAFETY:
> -            // - `dma_obj`'s size is at least `data.len()`.
> -            // - We have just created this object and there is no other user at this stage.
> -            unsafe {
> -                core::ptr::copy_nonoverlapping(
> -                    data.as_ptr(),
> -                    dma_obj.dma.start_ptr_mut(),
> -                    data.len(),
> -                );
> -            }
> -
> +            // SAFETY: We have just allocated the DMA memory, we are the only users and
> +            // we haven't made the device aware of the handle yet.
> +            unsafe { dma_obj.write(data, 0)? }

This doesn't build for me:

    error[E0277]: the `?` operator can only be used in a closure that returns `Result` or `Option` (or another type that implements `core::ops::FromResidual`)
      --> ../drivers/gpu/nova-core/dma.rs:31:44
      |
    28 |         Self::new(dev, data.len()).map(|mut dma_obj| {
      |                                        ------------- this function should return `Result` or `Option` to accept `?`
    ...
    31 |             unsafe { dma_obj.write(data, 0)? }
      |                                            ^ cannot use the `?` operator in a closure that returns `dma::DmaObject`

Could you double-check? I guess you will need to change the `map` into
`and_then`.