linux-kernel - Re: [PATCH 1/8] dma-mapping: add DMA_ATTR_LOW

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Message-ID: <20200925162003.GA18879@lst.de>
Date:   Fri, 25 Sep 2020 18:20:03 +0200
From:   Christoph Hellwig <hch@....de>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     Linux IOMMU <iommu@...ts.linux-foundation.org>,
        linux-media@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>, Christoph Hellwig <hch@....de>,
        Sylwester Nawrocki <snawrocki@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Tomasz Figa <tfiga@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] dma-mapping: add DMA_ATTR_LOW_ADDRESS attribute

>  #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.