Whilst blindly assuming the worst case for segment boundaries and aligning every segment individually is safe from the point of view of respecting the device's parameters, it is also undeniably a waste of IOVA space. Futhermore, the knock-on effects of more pages than necessary being exposed to device access, additional overhead in page table updates and TLB invalidations, etc., are even more undesirable.
Improve matters by taking the actual boundary mask into account to actively detect the cases in which we really do need to adjust a segment, and avoid wasting space in the remainder. Tested-by: Yong Wu <yong...@mediatek.com> Signed-off-by: Robin Murphy <robin.mur...@arm.com> --- Minor change: removed the now-redundant null check on prev, since we no longer dereference it unconditionally and pad_len is guaranteed to be zero the first time around. drivers/iommu/dma-iommu.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c index 3a20db4..427fdc1 100644 --- a/drivers/iommu/dma-iommu.c +++ b/drivers/iommu/dma-iommu.c @@ -441,6 +441,7 @@ int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg, struct scatterlist *s, *prev = NULL; dma_addr_t dma_addr; size_t iova_len = 0; + unsigned long mask = dma_get_seg_boundary(dev); int i; /* @@ -452,6 +453,7 @@ int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg, for_each_sg(sg, s, nents, i) { size_t s_offset = iova_offset(iovad, s->offset); size_t s_length = s->length; + size_t pad_len = (mask - iova_len + 1) & mask; sg_dma_address(s) = s->offset; sg_dma_len(s) = s_length; @@ -460,15 +462,13 @@ int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg, 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. + * With a single size-aligned IOVA allocation, no segment risks + * crossing the boundary mask unless the total size exceeds + * the mask itself. The simple way to maintain alignment when + * that does happen is to pad the previous segment to end at the + * next boundary, at the cost of 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); + if (pad_len && pad_len < s_length - 1) { prev->length += pad_len; iova_len += pad_len; } -- 1.9.1 _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu