Re: [PATCH 1/2] rust: Add dma_set_mask() and dma_set_coherent_mask() bindings

["Danilo Krummrich" <dakr@kernel.org>]

On Tue Jul 8, 2025 at 10:40 AM CEST, Danilo Krummrich wrote:
> On Tue Jul 8, 2025 at 8:04 AM CEST, Alistair Popple wrote:
>> Add bindings to allow setting the DMA masks for both a generic device
>> and a PCI device.
>
> Nice coincidence, I was about to get back to this. I already implemented this in
> a previous patch [1], but didn't apply it yet.
>
> I think the approach below is thought a bit too simple:
>
>   (1) We want the DMA mask methods to be implemented by a trait in dma.rs.
>       Subsequently, the trait should only be implemented by bus devices where
>       the bus actually supports DMA. Allowing to set the DMA mask on any device
>       doesn't make sense.

Forgot to mention, another reason for a trait is that we can also use it as a
trait bound on dma::CoherentAllocation::new(), such that people can't pass
arbitrary devices to dma::CoherentAllocation::new(), but only those that
actually sit on a DMA capable bus.

>
>   (2) We need to consider that with this we do no prevent
>       dma_set_coherent_mask() to concurrently with dma_alloc_coherent() (not
>       even if we'd add a new `Probe` device context).
>
> (2) is the main reason why I didn't follow up yet. So far I haven't found a nice
>     solution for a sound API that doesn't need unsafe.
>
> One thing I did consider was to have some kind of per device table (similar to
> the device ID table) for drivers to specify the DMA mask already at compile
> time. However, I'm pretty sure there are cases where the DMA mask has to derived
> dynamically from probe().
>
> I think I have to think a bit more about it.
>
> [1] https://lore.kernel.org/all/20250317185345.2608976-7-abdiel.janulgue@gmail.com/