Re: [PATCH v4 13/15] iommu/dma: Force bouncing if the size is not cacheline-aligned

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, May 19, 2023 at 03:02:24PM +0100, Catalin Marinas wrote:
> On Fri, May 19, 2023 at 01:29:38PM +0100, Robin Murphy wrote:
> > On 2023-05-18 18:34, Catalin Marinas wrote:
> > > +				sg_dma_mark_bounced(sg);
> > 
> > I'd prefer to have iommu_dma_map_sg_swiotlb() mark the segments, since
> > that's in charge of the actual bouncing. Then we can fold the alignment
> > check into dev_use_swiotlb() (with the dev_is_untrusted() condition taking
> > priority), and sync/unmap can simply rely on sg_is_dma_bounced() alone.
> 
> With this patch we only set the SG_DMA_BOUNCED on the first element of
> the sglist. Do you want to set this flag only on individual elements
> being bounced? It makes some sense in principle but the
> iommu_dma_unmap_sg() path would need to scan the list again to decide
> whether to go the swiotlb path.
> 
> If we keep the SG_DMA_BOUNCED flag only on the first element, I can
> change it to your suggestion, assuming I understood it.

Can one call:

	iommu_dma_map_sg(sg, nents);
	...
	iommu_dma_unmap_sg(sg + n, nents - n);

(i.e. unmap it in multiple steps)

If yes, setting SG_DMA_BOUNCED on the first element only won't work. I
don't find this an unlikely scenario, so we maybe we do have to walk the
list again in unmap to search for the flag.

-- 
Catalin