[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;
+}
