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 11:36 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>; m.szyprowski@xxxxxxxxxxx;
> 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 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