linux-kernel - Re: [PATCH v2] dma-mapping: benchmark: pretend DMA is transmitting

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Date:   Fri, 5 Feb 2021 11:36:27 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     Christoph Hellwig <hch@....de>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.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.