[PATCH v6 1/3] iommu: Implement common IOMMU ops for DMA mapping

[Yong Wu]

On Thu, 2015-10-01 at 20:13 +0100, Robin Murphy wrote:
[...]
> +/*
> + * The DMA API client is passing in a scatterlist which could describe
> + * any old buffer layout, but the IOMMU API requires everything to be
> + * aligned to IOMMU pages. Hence the need for this complicated bit of
> + * impedance-matching, to be able to hand off a suitably-aligned list,
> + * but still preserve the original offsets and sizes for the caller.
> + */
> +int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
> +		int nents, int prot)
> +{
> +	struct iommu_domain *domain = iommu_get_domain_for_dev(dev);
> +	struct iova_domain *iovad = domain->iova_cookie;
> +	struct iova *iova;
> +	struct scatterlist *s, *prev = NULL;
> +	dma_addr_t dma_addr;
> +	size_t iova_len = 0;
> +	int i;
> +
> +	/*
> +	 * Work out how much IOVA space we need, and align the segments to
> +	 * IOVA granules for the IOMMU driver to handle. With some clever
> +	 * trickery we can modify the list in-place, but reversibly, by
> +	 * hiding the original data in the as-yet-unused DMA fields.
> +	 */
> +	for_each_sg(sg, s, nents, i) {
> +		size_t s_offset = iova_offset(iovad, s->offset);
> +		size_t s_length = s->length;
> +
> +		sg_dma_address(s) = s->offset;
> +		sg_dma_len(s) = s_length;
> +		s->offset -= s_offset;
> +		s_length = iova_align(iovad, s_length + s_offset);
> +		s->length = s_length;
> +
> +		/*
> +		 * The simple way to avoid the rare case of a segment
> +		 * crossing the boundary mask is to pad the previous one
> +		 * to end at a naturally-aligned IOVA for this one's size,
> +		 * at the cost of potentially over-allocating a little.
> +		 */
> +		if (prev) {
> +			size_t pad_len = roundup_pow_of_two(s_length);
> +
> +			pad_len = (pad_len - iova_len) & (pad_len - 1);
> +			prev->length += pad_len;

Hi Robin,
      While our v4l2 testing, It seems that we met a problem here.
      Here we update prev->length again, Do we need update
sg_dma_len(prev) again too?

      Some function like vb2_dc_get_contiguous_size[1] always get
sg_dma_len(s) to compare instead of s->length. so it may break
unexpectedly while sg_dma_len(s) is not same with s->length.

[1]:
http://lxr.free-electrons.com/source/drivers/media/v4l2-core/videobuf2-dma-contig.c#L70


> +			iova_len += pad_len;
> +		}
> +
> +		iova_len += s_length;
> +		prev = s;
> +	}
> +
> +	iova = __alloc_iova(iovad, iova_len, dma_get_mask(dev));
> +	if (!iova)
> +		goto out_restore_sg;
> +
> +	/*
> +	 * We'll leave any physical concatenation to the IOMMU driver's
> +	 * implementation - it knows better than we do.
> +	 */
> +	dma_addr = iova_dma_addr(iovad, iova);
> +	if (iommu_map_sg(domain, dma_addr, sg, nents, prot) < iova_len)
> +		goto out_free_iova;
> +
> +	return __finalise_sg(dev, sg, nents, dma_addr);
> +
> +out_free_iova:
> +	__free_iova(iovad, iova);
> +out_restore_sg:
> +	__invalidate_sg(sg, nents);
> +	return 0;
> +}
> +