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

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@xxxxxx]
> Sent: Friday, February 5, 2021 10:21 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: m.szyprowski@xxxxxxxxxxx; hch@xxxxxx; robin.murphy@xxxxxxx;
> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> linuxarm@xxxxxxxxxxxxx
> 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