linux-kernel - Re: [PATCH 1/2] MIPS: Define pgprot_dmacoherent according to coherentio status

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Date:   Mon, 13 Jan 2020 16:12:53 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>
Cc:     Christoph Hellwig <hch@....de>, linux-mips@...r.kernel.org,
        chenhc@...ote.com, paul.burton@...s.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] MIPS: Define pgprot_dmacoherent according to
 coherentio status

On Mon, Jan 13, 2020 at 11:11:58PM +0800, Jiaxun Yang wrote:
> 
> 
> 于 2020年1月13日 GMT+08:00 下午10:58:20, Christoph Hellwig <hch@....de> 写到:
> >On Mon, Jan 13, 2020 at 10:07:04PM +0800, Jiaxun Yang wrote:
> >> For MIPS chips that support coherentio DMA, it's always safe
> >> to make DMA requests cached.
> >
> >For DMA coherent devices pgprot_dmacoherent isn't even used, so this
> >doesn't make sense.
> 
> Hi,
> 
> I'm supposed to use it later in drm/ttm
>  ttm_bo_util to overwrite pgprot for TTM_PL_SYSTEM.
> And I wish this pgropt can always represent dmacoherent pgropt.
> Would it be fine?

No code outside the core DMA code has any business using
pgprot_dmacoherent.