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SubjectRe: [PATCH 3/6] dma-mapping: add a dma_alloc_noncontiguous API
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[Archaeology ensues...]

On 2021-03-01 08:52, Christoph Hellwig wrote:
[...]
> +static struct sg_table *alloc_single_sgt(struct device *dev, size_t size,
> + enum dma_data_direction dir, gfp_t gfp)
> +{
> + struct sg_table *sgt;
> + struct page *page;
> +
> + sgt = kmalloc(sizeof(*sgt), gfp);
> + if (!sgt)
> + return NULL;
> + if (sg_alloc_table(sgt, 1, gfp))
> + goto out_free_sgt;
> + page = __dma_alloc_pages(dev, size, &sgt->sgl->dma_address, dir, gfp);
> + if (!page)
> + goto out_free_table;
> + sg_set_page(sgt->sgl, page, PAGE_ALIGN(size), 0);
> + sg_dma_len(sgt->sgl) = sgt->sgl->length;
> + return sgt;
> +out_free_table:
> + sg_free_table(sgt);
> +out_free_sgt:
> + kfree(sgt);
> + return NULL;
> +}
> +
> +struct sg_table *dma_alloc_noncontiguous(struct device *dev, size_t size,
> + enum dma_data_direction dir, gfp_t gfp, unsigned long attrs)
> +{
> + const struct dma_map_ops *ops = get_dma_ops(dev);
> + struct sg_table *sgt;
> +
> + if (WARN_ON_ONCE(attrs & ~DMA_ATTR_ALLOC_SINGLE_PAGES))
> + return NULL;
> +
> + if (ops && ops->alloc_noncontiguous)
> + sgt = ops->alloc_noncontiguous(dev, size, dir, gfp, attrs);
> + else
> + sgt = alloc_single_sgt(dev, size, dir, gfp);
> +
> + if (sgt) {
> + sgt->nents = 1;
> + debug_dma_map_sg(dev, sgt->sgl, sgt->orig_nents, 1, dir);

It turns out this is liable to trip up DMA_API_DEBUG_SG (potentially
even in the alloc_single_sgt() case), since we've filled in sgt without
paying attention to the device's segment boundary/size parameters.

Now, it would be entirely possible to make the allocators "properly"
partition the pages into multiple segments per those constraints, but
given that there's no actual dma_map_sg() operation involved, and AFAIR
the intent here is really only to describe a single DMA-contiguous
buffer as pages, rather than represent a true scatter-gather operation,
I'm now wondering whether it makes more sense to just make dma-debug a
bit cleverer instead. Any other opinions?

Thanks,
Robin.

> + }
> + return sgt;
> +}

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