linux-kernel - Re: [PATCH v6 2/2] rust: add dma coherent allocator abstraction.

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Message-Id: <F62F742D-B373-43D1-B527-9B9D3E5DB211@collabora.com>
Date: Thu, 5 Dec 2024 13:41:38 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>
Cc: Robin Murphy <robin.murphy@....com>,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>,
 Valentin Obst <kernel@...entinobst.de>,
 "open list:RUST" <rust-for-linux@...r.kernel.org>,
 open list <linux-kernel@...r.kernel.org>,
 Christoph Hellwig <hch@....de>,
 Marek Szyprowski <m.szyprowski@...sung.com>,
 airlied@...hat.com,
 "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v6 2/2] rust: add dma coherent allocator abstraction.

Hi Abdiel,

> On 5 Dec 2024, at 11:25, Abdiel Janulgue <abdiel.janulgue@...il.com> wrote:
> 
> On 05/12/2024 15:07, Robin Murphy wrote:
>>> +    pub fn alloc_coherent(
>>> +        dev: &Device,
>>> +        count: usize,
>>> +        flags: kernel::alloc::Flags,
>> Might it be worth adding at least a placeholder type and argument for "attrs" as well, so there's less churn involved in coming back and adding it later? Or is the intention to have a separate dma_alloc_attrs() constructor for such users?
> 
> It certainly is possible to include attrs in the constructor using Option:
> ```
>    pub fn alloc_coherent(
>        dev: &Device,
>        count: usize,
>        flags: kernel::alloc::Flags,
>        attrs: Option<u64>,
>    ) -> Result<CoherentAllocation<T>> {
> 
> <cut>
>        let ret = unsafe {
>            bindings::dma_alloc_attrs(
>                dev.as_raw(),
>                size,
>                &mut dma_handle, flags.as_raw(),
>                attrs.unwrap_or(0),
>            )
>        };
> <cut>
>    }
> 
> ```
> So the signature of the constructor could be:
> 
> let c = CoherentAllocation::alloc_coherent(dev, 4, GFP_KERNEL, None)?;
> 
> Andreas, Miguel, Daniel, any thoughts?
> 
> Regards,
> Abdiel
> 

Instead of a raw u64, you should take inspiration from the work in the Alloc crate and elsewhere to define
a proper `flags` type, which you can name `attr` in this case.

— Daniel