linux-kernel - Re: [PATCH v6 08/15] swiotlb: Bounce data from/to restricted DMA pool if available

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Date:   Mon, 10 May 2021 17:05:16 +0200
From:   Christoph Hellwig <hch@....de>
To:     Claire Chang <tientzu@...omium.org>
Cc:     Rob Herring <robh+dt@...nel.org>, mpe@...erman.id.au,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
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        boris.ostrovsky@...cle.com, jgross@...e.com,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        benh@...nel.crashing.org, paulus@...ba.org,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        sstabellini@...nel.org, Robin Murphy <robin.murphy@....com>,
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Subject: Re: [PATCH v6 08/15] swiotlb: Bounce data from/to restricted DMA
 pool if available

> +static inline bool is_dev_swiotlb_force(struct device *dev)
> +{
> +#ifdef CONFIG_DMA_RESTRICTED_POOL
> +	if (dev->dma_io_tlb_mem)
> +		return true;
> +#endif /* CONFIG_DMA_RESTRICTED_POOL */
> +	return false;
> +}
> +

>  	/* If SWIOTLB is active, use its maximum mapping size */
>  	if (is_swiotlb_active(dev) &&
> -	    (dma_addressing_limited(dev) || swiotlb_force == SWIOTLB_FORCE))
> +	    (dma_addressing_limited(dev) || swiotlb_force == SWIOTLB_FORCE ||
> +	     is_dev_swiotlb_force(dev)))

This is a mess.  I think the right way is to have an always_bounce flag
in the io_tlb_mem structure instead.  Then the global swiotlb_force can
go away and be replace with this and the fact that having no
io_tlb_mem structure at all means forced no buffering (after a little
refactoring).