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191aee58b6
The description says: Cause IO segments sent to a device for DMA to be merged virtually by the IOMMU when they happen to have been allocated contiguously. This doesn't add pressure to the IOMMU allocator. However, some drivers don't support getting large merged segments coming back from *_map_sg(). Most drivers don't have this problem; it is safe to say Y here. It's out of date. Long ago, drivers didn't have a way to tell IOMMUs about their segment length limit (that is, the maximum segment length that they can handle). So IOMMUs merged as many segments as possible and gave too large segments to drivers. dma_get_max_seg_size() was introduced to solve the above problem. Device drives can use the API to tell IOMMU about the maximum segment length that they can handle. In addition, the default limit (64K) should be safe for everyone. So this config option seems to be unnecessary. Note that this config option just enables users to disable the virtual merging by default. Users can still disable the virtual merging by the boot parameter. Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org> |
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