linux-kernel - Re: [PATCH 3/4] dma-mapping: add a dma_ops

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Date:   Sat, 18 Apr 2020 14:42:05 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Alexey Kardashevskiy <aik@...abs.ru>,
        linuxppc-dev@...ts.ozlabs.org, Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] dma-mapping: add a dma_ops_bypass flag to struct
 device

Hi Christoph,

On Tue, Apr 14, 2020 at 02:25:05PM +0200, Christoph Hellwig wrote:
> +static inline bool dma_map_direct(struct device *dev,
> +		const struct dma_map_ops *ops)
> +{
> +	if (likely(!ops))
> +		return true;
> +	if (!dev->dma_ops_bypass)
> +		return false;
> +
> +	return min_not_zero(*dev->dma_mask, dev->bus_dma_limit) >=
> +			    dma_direct_get_required_mask(dev);

Why is the dma-mask check done here? The dma-direct code handles memory
outside of the devices dma-mask with swiotlb, no?

I also don't quite get what the difference between setting the
dma_ops_bypass flag non-zero and setting ops to NULL is.


	Joerg