linux-kernel - Re: [PATCH 1/2] nova-core: Solve mentions of `CoherentAllocation` improvements [COHA]

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Message-ID: <a7ccda73-2c40-419c-a7c3-3155739648d0@gmail.com>
Date: Thu, 16 Oct 2025 23:46:28 +0200
From: Daniel del Castillo <delcastillodelarosadaniel@...il.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: 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>,
 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
Subject: Re: [PATCH 1/2] nova-core: Solve mentions of `CoherentAllocation`
 improvements [COHA]

Hi Danilo,

On 10/15/25 22:04, Danilo Krummrich wrote:
>> diff --git a/drivers/gpu/nova-core/dma.rs b/drivers/gpu/nova-core/dma.rs
>> index 94f44bcfd748..639a99cf72c4 100644
>> --- a/drivers/gpu/nova-core/dma.rs
>> +++ b/drivers/gpu/nova-core/dma.rs
>> @@ -25,21 +25,11 @@ 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(),
>> -                );
>> -            }
>> -
>> -            dma_obj
>> -        })
>> +        let mut dma_obj = Self::new(dev, data.len())?;
>> +        // SAFETY: We have just created this object and there is no other user at this stage.
> 
> The safety comment should rather confirm that it is guaranteed that the device
> won't access this memory concurrently.

I actually don't know how this is guaranteed. It wasn't explicitly
explained before here, although unless I'm mistaken it was already a
requirement. Could you help me? I guess it's related to the already
mentioned fact that we just allocated this DMA memory and the device
isn't yet initialized?

Thanks,
Daniel