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

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Message-ID: <51200cc6-042a-4e83-81b0-720385c67bb5@gmail.com>
Date: Sun, 2 Nov 2025 15:56:16 +0100
From: Daniel del Castillo <delcastillodelarosadaniel@...il.com>
To: Alexandre Courbot <acourbot@...dia.com>,
 Danilo Krummrich <dakr@...nel.org>, 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

Hi Alexandre,

On 11/1/25 12:46, Alexandre Courbot wrote:
> 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`.

You are totally right. I'm not sure what happened. I'll fix it. Thanks!