linux-kernel - Re: [PATCH 2/3] dma: Introduce dma_max_mapping

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Date:   Tue, 15 Jan 2019 14:37:54 +0100
From:   Christoph Hellwig <hch@....de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        virtualization@...ts.linux-foundation.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jfehlig@...e.com,
        jon.grimm@....com, brijesh.singh@....com, jroedel@...e.de
Subject: Re: [PATCH 2/3] dma: Introduce dma_max_mapping_size()

> +size_t dma_direct_max_mapping_size(struct device *dev)
> +{
> +	/*
> +	 * Return the minimum of the direct DMA limit and the SWIOTLB limit.
> +	 * Since direct DMA has no limit, it is fine to just return the SWIOTLB
> +	 * limit.
> +	 */
> +	return swiotlb_max_mapping_size(dev);

Well, if we don't actually use the swiotlb buffers despite it being
compiled in or even allocated we don't need the limit.