-----Original Message----- From: Christoph Hellwig [mailto:hch@lst.de] Sent: Friday, February 5, 2021 10:21 PM To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com> Cc: m.szyprowski@samsung.com; hch@lst.de; robin.murphy@arm.com; iommu@lists.linux-foundation.org; linux-kernel@vger.kernel.org; linuxarm@openeuler.org Subject: Re: [PATCH v2] dma-mapping: benchmark: pretend DMA is transmitting
On Fri, Feb 05, 2021 at 03:00:35PM +1300, Barry Song wrote:
+ __u32 dma_trans_ns; /* time for DMA transmission in ns */ __u64 expansion[10]; /* For future use */
We need to keep the struct size, so the expansion field needs to shrink by the equivalent amount of data that is added in dma_trans_ns.
Unfortunately I didn't put a rsv u32 field after dma_dir in the original patch. There were five 32bits data before expansion[]: struct map_benchmark { __u64 avg_map_100ns; /* average map latency in 100ns */ __u64 map_stddev; /* standard deviation of map latency */ __u64 avg_unmap_100ns; /* as above */ __u64 unmap_stddev; __u32 threads; /* how many threads will do map/unmap in parallel */ __u32 seconds; /* how long the test will last */ __s32 node; /* which numa node this benchmark will run on */ __u32 dma_bits; /* DMA addressing capability */ __u32 dma_dir; /* DMA data direction */ __u64 expansion[10]; /* For future use */ }; My bad. That was really silly. I should have done the below from the first beginning: struct map_benchmark { __u64 avg_map_100ns; /* average map latency in 100ns */ __u64 map_stddev; /* standard deviation of map latency */ __u64 avg_unmap_100ns; /* as above */ __u64 unmap_stddev; __u32 threads; /* how many threads will do map/unmap in parallel */ __u32 seconds; /* how long the test will last */ __s32 node; /* which numa node this benchmark will run on */ __u32 dma_bits; /* DMA addressing capability */ __u32 dma_dir; /* DMA data direction */ __u32 rsv; __u64 expansion[10]; /* For future use */ }; So on 64bit system, this patch doesn't change the length of struct as the new added u32 just fill the gap between dma_dir and expansion. For 32bit system, this patch increases 4 bytes in the length. I can keep the struct size unchanged by changing the struct to struct map_benchmark { __u64 avg_map_100ns; /* average map latency in 100ns */ __u64 map_stddev; /* standard deviation of map latency */ __u64 avg_unmap_100ns; /* as above */ __u64 unmap_stddev; __u32 threads; /* how many threads will do map/unmap in parallel */ __u32 seconds; /* how long the test will last */ __s32 node; /* which numa node this benchmark will run on */ __u32 dma_bits; /* DMA addressing capability */ __u32 dma_dir; /* DMA data direction */ __u32 dma_trans_ns; /* time for DMA transmission in ns */ __u32 exp; /* For future use */ __u64 expansion[9]; /* For future use */ }; But the code is really ugly now. Thanks Barry