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The > per-device limit from dma_set_max_seg_size() is ignored, so contiguous > page segments can be longer than the declared max segment size, > potentially causing problems for future drivers that use this > abstraction. > > nova-core declares an unlimited segment size, so this does not change > its behavior. I guess what this last paragraph wants to state is that no user is affected by this patch? There is another subtle user though: the Rust DMA sample. In any case, this patch without patch 1 wouldn't break either of those (only waste a bit more memory in SG entries with nova-core), so maybe we can skip it. > > Fixes: 05aa6fb1c21d ("rust: scatterlist: Add abstraction for sg_table") > Signed-off-by: Matteo Kloiber > --- > rust/helpers/dma.c | 5 +++++ > rust/kernel/scatterlist.rs | 12 ++++++++++-- > 2 files changed, 15 insertions(+), 2 deletions(-) > > diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c > index 9fbeb507b08c..ff8f24dae9df 100644 > --- a/rust/helpers/dma.c > +++ b/rust/helpers/dma.c > @@ -49,3 +49,8 @@ __rust_helper void rust_helper_dma_set_max_seg_size(str= uct device *dev, > { > dma_set_max_seg_size(dev, size); > } > + > +__rust_helper unsigned int rust_helper_dma_get_max_seg_size(struct devic= e *dev) > +{ > + return dma_get_max_seg_size(dev); > +} > diff --git a/rust/kernel/scatterlist.rs b/rust/kernel/scatterlist.rs > index b83c468b5c63..d677dcbe7aac 100644 > --- a/rust/kernel/scatterlist.rs > +++ b/rust/kernel/scatterlist.rs > @@ -350,15 +350,23 @@ fn new( > page_vec.push(page.as_ptr(), flags)?; > } > =20 > + // Cap segments at both the DMA mapping-path limit and the devic= e's declared > + // max segment size. > + // > // `dma_max_mapping_size` returns `size_t`, but `sg_alloc_table_= from_pages_segment()` takes > // an `unsigned int`. > // > // SAFETY: `dev.as_raw()` is a valid pointer to a `struct device= `. > - let max_segment =3D match unsafe { bindings::dma_max_mapping_siz= e(dev.as_raw()) } { > + let max_mapping =3D match unsafe { bindings::dma_max_mapping_siz= e(dev.as_raw()) } { > 0 =3D> u32::MAX, > - max_segment =3D> u32::try_from(max_segment).unwrap_or(u32::M= AX), > + max_mapping =3D> u32::try_from(max_mapping).unwrap_or(u32::M= AX), nit: if we rename this variable, let's name it `max_mapping_size` for consistency. > }; > =20 > + // SAFETY: `dev.as_raw()` is a valid pointer to a `struct device= `. > + let max_seg_size =3D unsafe { bindings::dma_get_max_seg_size(dev= .as_raw()) }; > + > + let max_segment =3D max_mapping.min(max_seg_size); This looks like the correct thing to do indeed. Reviewed-by: Alexandre Courbot (the following is a related thought, not a request for this patch) It also means that without patch 1, nova-core would split the firmware into hundreds of 64KB SG entries, which is not breaking but still something we want to avoid. The correct fix is to make sure that `dma_set_max_seg_size` is called by the driver, and while we are at it we also want every driver to call `dma_set_mask_and_coherent`. Ideally we would use the type system to make sure that both functions are called before any DMA operation can take place (using a safe interface), but I'm not quite sure yet how we can do this.