linux-kernel - Re: [PATCH 2/2] arm-nommu: Add use_reserved_mem() to check if device support reserved memory

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Date:   Tue, 9 Jun 2020 19:34:55 +0200
From:   Christoph Hellwig <hch@....de>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     Christoph Hellwig <hch@....de>,
        dillon min <dillon.minfei@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        linux@...linux.org.uk, Kate Stewart <kstewart@...uxfoundation.org>,
        allison@...utok.net, info@...ux.net, tglx@...utronix.de,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-stm32@...md-mailman.stormreply.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] arm-nommu: Add use_reserved_mem() to check if
 device support reserved memory

On Tue, Jun 09, 2020 at 05:25:04PM +0100, Vladimir Murzin wrote:
> Even though commit mentions ARM, I do not see how mmap would continue
> to work for NOMMU with dma-direct. ARM NOMMU needs it's own DMA operations
> only in cases where caches are implemented or active, in other cases it
> fully relies on dma-direct.

> It looks to me that we either should provide NOMMU variant for mmap in
> dma/direct or (carefully) fix dma/mapping.

I think dma-direct is the right place, the common helpers in
dma/mapping.c are basically the red headed stepchilds for misc
IOMMU drivers not covered by dma-iommu only.