linux-kernel - Re: [PATCH 6/6] MIPS: document mixing "slightly different CCAs"

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Message-ID: <20190827100310.p2o5y3aklcoti74z@pburton-laptop>
Date:   Tue, 27 Aug 2019 10:03:16 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Christoph Hellwig <hch@....de>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        Guan Xuetao <gxt@....edu.cn>,
        Shawn Anastasio <shawn@...stas.io>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        James Hogan <jhogan@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] MIPS: document mixing "slightly different CCAs"

Hi Christoph,

On Mon, Aug 26, 2019 at 03:25:53PM +0200, Christoph Hellwig wrote:
> Based on an email from Paul Burton, quoting section 4.8 "Cacheability and
> Coherency Attributes and Access Types" of "MIPS Architecture Volume 1:
> Introduction to the MIPS32 Architecture" (MD00080, revision 6.01).
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

Acked-by: Paul Burton <paul.burton@...s.com>

Thanks,
    Paul

> ---
>  arch/mips/Kconfig | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index fc88f68ea1ee..aff1cadeea43 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -1119,6 +1119,13 @@ config DMA_PERDEV_COHERENT
>  
>  config DMA_NONCOHERENT
>  	bool
> +	#
> +	# MIPS allows mixing "slightly different" Cacheability and Coherency
> +	# Attribute bits.  It is believed that the uncached access through
> +	# KSEG1 and the implementation specific "uncached accelerated" used
> +	# by pgprot_writcombine can be mixed, and the latter sometimes provides
> +	# significant advantages.
> +	#
>  	select ARCH_HAS_DMA_WRITE_COMBINE
>  	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
>  	select ARCH_HAS_UNCACHED_SEGMENT
> -- 
> 2.20.1
>