-----Original Message----- From: Christoph Hellwig [mailto:hch@lst.de] Sent: Friday, February 5, 2021 11:36 PM To: Song Bao Hua (Barry Song) song.bao.hua@hisilicon.com Cc: Christoph Hellwig hch@lst.de; m.szyprowski@samsung.com; 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 10:32:26AM +0000, Song Bao Hua (Barry Song) wrote:
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.
Thats why we usually use __u8 fields for reserved field. You might consider just switching to that instead while you're at it. I guess we'll just have to get the addition into 5.11 then to make sure we don't release a kernel with the alignment fix.
I assume there is no need to keep the same size with 5.11-rc, so could change 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 */ __u8 expansion[84]; /* For future use */ };
This won't increase size on 64bit system, but it increases 4bytes on 32bits system comparing to 5.11-rc. How do you think about it?
Thanks Barry