On 26/10/15 13:44, Yong Wu wrote:
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.
This is just tweaking the faked-up length that we hand off to iommu_map_sg() (see also the iova_align() above), to trick it into bumping this segment up to a suitable starting IOVA. The real length at this point is stashed in sg_dma_len(s), and will be copied back into s->length in __finalise_sg(), so both will hold the same true length once we return to the caller.
Yes, it does mean that if you have a list where the segment lengths are page aligned but not monotonically decreasing, e.g. {64k, 16k, 64k}, then you'll still end up with a gap between the second and third segments, but that's fine because the DMA API offers no guarantees about what the resulting DMA addresses will be (consider the no-IOMMU case where they would each just be "mapped" to their physical address). If that breaks v4l, then it's probably v4l's DMA API use that needs looking at (again).
Robin.
[1]: http://lxr.free-electrons.com/source/drivers/media/v4l2-core/videobuf2-dma-contig.c#L70
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