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

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Mostafa Saleh <smostafa@google.com> writes:

> 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().
>

There is a possibility that we may support io_tlb_mem with cc_shared =
false in the future. As a result, only swiotlb_map() knows which type of
bounce buffer was used, making it the only place where the attributes
can be updated correctly.

-aneesh