linux-kernel - Re: [PATCH v6 3/3] tools/dma: Add dma_map

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Message-ID: <CAGsJ_4xERKq4=iQPjP6+=7p4vdCUpfKB7hp7_QDOwUxn+viG5A@mail.gmail.com>
Date: Fri, 30 Jan 2026 17:16:08 +0800
From: Barry Song <21cnbao@...il.com>
To: Qinxin Xia <xiaqinxin@...wei.com>
Cc: jonathan.cameron@...wei.com, wangzhou1@...ilicon.com, 
	iommu@...ts.linux.dev, prime.zeng@...wei.com, fanghao11@...wei.com, 
	linux-kernel@...r.kernel.org, linuxarm@...wei.com
Subject: Re: [PATCH v6 3/3] tools/dma: Add dma_map_sg support

On Fri, Jan 30, 2026 at 4:38 PM Qinxin Xia <xiaqinxin@...wei.com> wrote:
>
>
>
> On 2026/1/26 10:51:11, Barry Song <21cnbao@...il.com> wrote:
> > On Mon, Jan 12, 2026 at 5:34 PM Qinxin Xia <xiaqinxin@...wei.com> wrote:
> >>
> >> Support for dma_map_sg, add option '-m' to distinguish mode.
> >>
> >> i) Users can set option '-m' to select mode:
> >>     DMA_MAP_BENCH_SINGLE_MODE=0, DMA_MAP_BENCH_SG_MODE:=1
> >>     (The mode is also show in the test result).
> >> ii) Users can set option '-g' to set sg_nents
> >>      (total count of entries in scatterlist)
> >>      the maximum number is 1024. Each of sg buf size is PAGE_SIZE.
> >>      e.g
> >>      [root@...alhost]# ./dma_map_benchmark -m 1 -g 8 -t 8 -s 30 -d 2
> >>      dma mapping mode: DMA_MAP_BENCH_SG_MODE
> >>      dma mapping benchmark: threads:8 seconds:30 node:-1
> >>      dir:FROM_DEVICE granule/sg_nents: 8
> >>      average map latency(us):1.4 standard deviation:0.3
> >>      average unmap latency(us):1.3 standard deviation:0.3
> >>      [root@...alhost]# ./dma_map_benchmark -m 0 -g 8 -t 8 -s 30 -d 2
> >>      dma mapping mode: DMA_MAP_BENCH_SINGLE_MODE
> >>      dma mapping benchmark: threads:8 seconds:30 node:-1
> >>      dir:FROM_DEVICE granule/sg_nents: 8
> >>      average map latency(us):1.0 standard deviation:0.3
> >>      average unmap latency(us):1.3 standard deviation:0.5
> >>
> >
> > What happens if m is set to 0 while g is set to 8?
> >
> > Thanks
> > Barry
>
> Hi Barry!
> m set '0' and g set '8', This means that 8 page_sizes are mapped at a
> time in single mode.
> As the comment for the struct map_benchmark definition says:
>
>   __u32 granule;  /* how many PAGE_SIZE will do map/unmap once a time */
>
> [root@...alhost xqx]# ./dma_map_benchmark -m 0 -g 8 -t 8 -s 30 -d 2
> dma mapping benchmark(SINGLE_MODE): threads:8 seconds:30 node:-1
> dir:FROM_DEVICE granule:8
> average map latency(us):0.2 standard deviation:0.1
> average unmap latency(us):4.3 standard deviation:1.4
>
> ======================================================
> The newly added sg mode reuses the -g option as sgnents and is described
> in the comments:
>         /*
>          * Set the number of scatterlist entries based on the granule.
>
>
>          * In SG mode, 'granule' represents the number of scatterlist
> entries.
>          * Each scatterlist entry corresponds to a single page.
>          */
>
> By the way, I've considered testing sgnents of different sizes, but it's
> not very easy to set for user parameters, so I set it with each
> scatterlist entry corresponds to a single page.

This is a bit odd. Ideally, we shouldn’t have a mixed definition
for a single variant, but since this is just a tool, it may be
acceptable.

That said, the documentation should at least be updated in
patches 2/3 and 3/3. As it stands, it still says:

    __u32 granule; /* how many PAGE_SIZE are mapped or unmapped
                     at a time */


>
> Thanks,
> Qinxin
>