Re: [PATCH v13 2/7] rust: add dma coherent allocator abstraction.

Jason Gunthorpe
2025-03-21T18:29:03Z

On Fri, Mar 21, 2025 at 06:34:53PM +0100, Danilo Krummrich wrote:
> On Fri, Mar 21, 2025 at 02:23:53PM -0300, Jason Gunthorpe wrote:
> > On Fri, Mar 07, 2025 at 01:06:19PM +0200, Abdiel Janulgue wrote:
> >
> > > + // SAFETY: Device pointer is guaranteed as valid by the type invariant on `Device`.
> > > + let ret = unsafe {
> > > + bindings::dma_alloc_attrs(
> > > + dev.as_raw(),
> > > + size,
> > > + &mut dma_handle,
> > > + gfp_flags.as_raw(),
> > > + dma_attrs.as_raw(),
> > > + )
> >
> > This is not the correct safety statement, the device must have a driver
> > bound to call this function, a struct device reference is not
> > sufficient.
> >
> > I belive Danilo was suggesting to ignore this unsafety for now, but if
> > so it should be documented correctly.
>
> If just landed patches [1], which are the foundation of addressing this issue.

Those patches say:

The context types can be extended as required, e.g. to limit availability of
certain (bus) device functions to probe().

Which is not an appropriate limitation for dma_alloc_coherent, we
expect it to be called outside probe in real drivers. Is there more to
that story?

Regardless, the safety comment should not be merged with incorrect
information. :\

> > Also think the use of devres here is going to be very problematic for
> > drivers to use as I said in other emails. :(
>
> In an earlier reply today in a different thread already gave you the link [2] of
> what we landed, which, besides explaining the situation, also makes clear that
> there is *no* Devres wrapper around a CoherentAllocation and why.

Yes, I see, I have so much email right now it is hard find all the
different versions of everything.

Jason