Re: [PATCH v8 12/23] dma: swiotlb: pass mapping attributes by reference

[Mostafa Saleh <smostafa@google.com>]

On Wed, Jul 29, 2026 at 02:35:36PM +0530, Aneesh Kumar K.V wrote:
> Mostafa Saleh <smostafa@google.com> writes:
> 
> > On Fri, Jul 17, 2026 at 11:34:30PM +0530, Aneesh Kumar K.V (Arm) wrote:
> >> Change swiotlb_tbl_map_single() to take the DMA mapping attributes by
> >> reference and update the direct callers accordingly.
> >> 
> >> This is a preparatory change for a follow-up patch which updates the
> >> attributes based on the selected swiotlb pool. Keeping the signature change
> >> separate makes the follow-up patch easier to review.
> >> 
> >
> > I do not understand the point of this, why would
> > swiotlb_tbl_map_single() force attrs over the caller which should
> > already know the context (whether DMA is shared or not) which case
> > would this be useful to force from the SWIOTLB code?
> >
> 
> The mapping request was for private memory, but the device could not use
> it because the dma_capable() (force_dma_unencrypted() == true) check
> failed. As a result, dma_direct_map_phys() allocated a swiotlb bounce
> buffer, which is shared. Since the DMA now targets the shared bounce
> buffer rather than the original private memory, the attributes must be
> updated to reflect the new shared mapping.
> 
> dma_addr = phys_to_dma(dev, phys);
> if (unlikely(!dma_capable(dev, dma_addr, size, true)))
> 	if (is_swiotlb_active(dev))
> 		return swiotlb_map(dev, phys, size, dir, attrs);
> 
> 

But it does not make sense to me to have it in such a low level
function, and it shouldn't change the attrs that was passed.

dma_direct_map_phys() is the right place to do this it should set
the correct attrs before calling swiotlb_map().

Thanks,
Mostafa

> -aneesh