linux-kernel - Re: [PATCH v11 2/3] rust: add dma coherent allocator abstraction.

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Message-ID: <ecf3752d-87ab-4b3b-8660-d827437b0ffb@gmail.com>
Date: Mon, 17 Feb 2025 19:37:43 +0200
From: Abdiel Janulgue <abdiel.janulgue@...il.com>
To: Robin Murphy <robin.murphy@....com>,
 Daniel Almeida <daniel.almeida@...labora.com>
Cc: rust-for-linux@...r.kernel.org, dakr@...nel.org, aliceryhl@...gle.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>, Trevor Gross <tmgross@...ch.edu>,
 Valentin Obst <kernel@...entinobst.de>,
 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 v11 2/3] rust: add dma coherent allocator abstraction.

On 17/02/2025 15:52, Robin Murphy wrote:
> On 15/02/2025 9:40 pm, Daniel Almeida wrote:
>> Hi Abdiel
>>
>> I noticed that there’s no API to call `dma_set_mask/ 
>> dma_set_coherent_mask`.
>>
>> This should probably be included, i.e.:
>>
>> ```
>> By default, the kernel assumes that your device can address 32-bits of 
>> DMA addressing.
>> For a 64-bit capable device, this needs to be increased, and for a 
>> device with limitations,
>> it needs to be decreased.
>> ```
> 
> Oh, good point (and I'm rather ashamed I missed that!)

Ok. Missed this part while working on the nova stub and hadn't had to 
use this interface yet, but yes will add this for the next revision.

> 
> FWIW we've been wanting to steer away from relying on the default mask 
> in new code, so it would be quite neat to actually enforce that 
> allocations fail if dma_coherent_mask hasn't been explicitly set 
> (assuming it's sufficiently cheap to keep a flag in the Device handle or 
> something like that - it's not the end of the world if it isn't practical).
> 
Thanks for the heads-up on what this interface is expected to look like!

Regards,
Abdiel