linux-kernel - Re: [PATCH v2] arc: Implement arch-specific dma_map

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Date:   Tue, 15 Nov 2016 15:45:35 +0000
From:   Ramiro Oliveira <Ramiro.Oliveira@...opsys.com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
CC:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v2] arc: Implement arch-specific dma_map_ops.mmap

On 11/3/2016 3:06 PM, Alexey Brodkin wrote:
> We used to use generic implementation of dma_map_ops.mmap which is
> dma_common_mmap() but that only worked for simpler cached mappings when
> vaddr = paddr.
> 
> If a driver requests uncached DMA buffer kernel maps it to virtual
> address so that MMU gets involved and page uncached status takes into
> account. In that case usage of dma_common_mmap() lead to mapping of
> vaddr to vaddr for user-space which is obviously wrong. For more detals
> please refer to verbose explanation here [1].
> 
> So here we implement our own version of mmap() which always deals
> with dma_addr and maps underlying memory to user-space properly
> (note that DMA buffer mapped to user-space is always uncached
> because there's no way to properly manage cache from user-space).
> 
> [1] https://lkml.org/lkml/2016/10/26/973
> 
> Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Vineet Gupta <vgupta@...opsys.com>
> Cc: <stable@...r.kernel.org>
> ---

Tested-by: Ramiro Oliveira <Ramiro.Oliveira@...opsys.com>