Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

[Abdiel Janulgue]

On 03/03/2025 13:30, Andreas Hindborg wrote:
> "Abdiel Janulgue" <abdiel.janulgue@xxxxxxxxx> writes:
>
> > Add a simple dma coherent allocator rust abstraction. Based on
> > Andreas Hindborg's dma abstractions from the rnvme driver, which
> > was also based on earlier work by Wedson Almeida Filho.
> >
> > Nacked-by: Christoph Hellwig <hch@xxxxxx>
> > Signed-off-by: Abdiel Janulgue <abdiel.janulgue@xxxxxxxxx>
> [...]
>
> > diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> > index 0640b7e115be..8f3808c8b7fe 100644
> > --- a/rust/helpers/helpers.c
> > +++ b/rust/helpers/helpers.c
> > @@ -13,6 +13,7 @@
> >   #include "build_bug.c"
> >   #include "cred.c"
> >   #include "device.c"
> > +#include "dma.c"
> >   #include "err.c"
> >   #include "fs.c"
> >   #include "io.c"
> > diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> > new file mode 100644
> > index 000000000000..b4dd5d411711
> > --- /dev/null
> > +++ b/rust/kernel/dma.rs
> > @@ -0,0 +1,421 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +//! Direct memory access (DMA).
> > +//!
> > +//! C header: [`include/linux/dma-mapping.h`](srctree/include/linux/dma-mapping.h)
> > +
> > +use crate::{
> > +    bindings, build_assert,
> > +    device::Device,
> > +    error::code::*,
> > +    error::Result,
> > +    transmute::{AsBytes, FromBytes},
> > +    types::ARef,
> > +};
> > +
> > +/// Inform the kernel about the device's DMA addressing capabilities. This will set the mask for
> > +/// both streaming and coherent APIs together.
> > +pub fn dma_set_mask_and_coherent(dev: &Device, mask: u64) -> i32 {
> > +    // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
> > +    unsafe { bindings::dma_set_mask_and_coherent(dev.as_raw(), mask) }
> > +}
> > +
> > +/// Same as `dma_set_mask_and_coherent`, but set the mask only for streaming mappings.
> > +pub fn dma_set_mask(dev: &Device, mask: u64) -> i32 {
> > +    // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
> > +    unsafe { bindings::dma_set_mask(dev.as_raw(), mask) }
> > +}
>
> I'm rebasing some of the dma pool code I'm using for NVMe on top of
> these patches, and I notice that these methods in the original code from
> way back (besides being on Device) has these methods return `Result`:
>
>      pub fn dma_set_mask(&self, mask: u64) -> Result {
>          let dev = self.as_raw();
>          let ret = unsafe { bindings::dma_set_mask(dev as _, mask) };
>          if ret != 0 {
>              Err(Error::from_errno(ret))
>          } else {
>              Ok(())
>          }
>      }
>
> Is there a reason for not returning a `Result` in this series?

Hi Andreas, the original dma_set_mask function got lost to me for some 
reason. But yes, this is indeed a better approach!

Regards,
Abdiel