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Date:   Fri, 25 Sep 2020 18:20:03 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Marek Szyprowski <m.szyprowski@samsung.com>
Cc:     Linux IOMMU <iommu@lists.linux-foundation.org>,
        linux-media@vger.kernel.org, linux-samsung-soc@vger.kernel.org,
        Robin Murphy <robin.murphy@arm.com>,
        Joerg Roedel <joro@8bytes.org>, Christoph Hellwig <hch@lst.de>,
        Sylwester Nawrocki <snawrocki@kernel.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Tomasz Figa <tfiga@chromium.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/8] dma-mapping: add DMA_ATTR_LOW_ADDRESS attribute
Message-ID: <20200925162003.GA18879@lst.de>
References: <20200925141218.13550-1-m.szyprowski@samsung.com> <CGME20200925141231eucas1p223c342cc989df23e4fc28d97fe3010c5@eucas1p2.samsung.com> <20200925141218.13550-2-m.szyprowski@samsung.com>
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>  #define DMA_ATTR_PRIVILEGED		(1UL << 9)
> +/*
> + * DMA_ATTR_LOW_ADDRESS: used to indicate that the buffer should be allocated
> + * at the lowest possible DMA address, usually just at the beginning of the
> + * DMA/IOVA address space ('first-fit' allocation algorithm).
> + */
> +#define DMA_ATTR_LOW_ADDRESS		(1UL << 10)

I think we need better comments explaining that this is best effort
and only applies to DMA API implementations that actually have an
allocatable IOVA space.