linux-kernel - Re: [PATCH 2/2] staging: vc04_services: Replace dmac_map_area with dmac_map

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Date:   Mon, 24 Oct 2016 09:56:35 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Greg KH <gregkh@...uxfoundation.org>, mzoran@...wfest.net
Cc:     daniels@...labora.com, noralf@...nnes.org, popcornmix@...il.com,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] staging: vc04_services: Replace dmac_map_area with dmac_map_sg

Greg KH <gregkh@...uxfoundation.org> writes:

> On Sun, Oct 23, 2016 at 10:29:32PM -0700, mzoran@...wfest.net wrote:
>> From: Michael Zoran <mzoran@...wfest.net>
>> 
>> The original arm implementation uses dmac_map_area which is not
>> portable.  Replace it with an architecture neutral version
>> which uses dma_map_sg.
>> 
>> As you can see that for larger page sizes, the dma_map_sg
>> implementation is faster then the original unportable dma_map_area
>> implementation.  
>> 
>> Test                       dmac_map_area   dma_map_page dma_map_sg
>> vchiq_test -b 4 10000      51us/iter       76us/iter    76us
>> vchiq_test -b 8 10000      70us/iter       82us/iter    91us
>> vchiq_test -b 16 10000     94us/iter       118us/iter   121us
>> vchiq_test -b 32 10000     146us/iter      173us/iter   187us
>> vchiq_test -b 64 10000     263us/iter      328us/iter   299us
>> vchiq_test -b 128 10000    529us/iter      631us/iter   595us
>> vchiq_test -b 256 10000    2285us/iter     2275us/iter  2001us
>> vchiq_test -b 512 10000    4372us/iter     4616us/iter  4123us
>> 
>> For message sizes >= 64KB, dma_map_sg is faster then dma_map_page.
>> 
>> For message size >= 256KB, the dma_map_sg is the fastest
>> implementation.
>
> What is the "normal" message size value when using this driver?

For the contents of a bulk transfer, the performance case you care about
is moving an image (video decode, camera, etc.), so on the order of 1MB.

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