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SubjectRE: [PATCH v2] dma-mapping: benchmark: pretend DMA is transmitting
Date


> -----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

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