Re: [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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