linux-kernel - Re: [PATCH mmc-next v3 3/3] mmc: sdhci-of-dwcmshc: solve 128MB DMA boundary limitation

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Date:   Mon, 30 Jul 2018 11:11:33 +0800
From:   Jisheng Zhang <Jisheng.Zhang@...aptics.com>
To:     Matthew Leon <matthewleon@...ux.com>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH mmc-next v3 3/3] mmc: sdhci-of-dwcmshc: solve 128MB DMA
 boundary limitation

On Mon, 30 Jul 2018 03:11:59 +0000
Matthew Leon <matthewleon@...ux.com> wrote:

>  >> Hey Jisheng,  
> 
> >Hi,  
> 
> >>  
> 
> >In LKML, we'd better not top post.  
> 
> Noted. My apologies.
> 
> >> Shouldn't we be splitting until all DMA blocks are less than 128M  
> boundary?
> >> I am a noob, but I think we should be prepared for boundaries that when
> >> split in two, will still be greater than 128M. Feel free to disagree but
> >> please explain why I may be wrong. Thank-you.  
> 
> >the limitation is "DMA addr can't span 128MB boundary" rather than "must be
> >less than 128MB", they are different.  
> 
> >And the max transfer size of one DMA desc is 64KB.  
> 
> >thanks  
> 
> I have misspoken. What if the DMA transfer size is 1024M? If we split in
> two, then we have 2 transfers, each of which span 512M. So wouldn't we need
> to split again to have 4 transfers, each of which span 128M?
> 

the max transfer size of each desc is 64KB, how could it be 1024MB?