linux-kernel - Re: [PATCH 1/2] dma-mapping: add a dma_ops

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Message-ID: <20200323172256.GB31269@lst.de>
Date:   Mon, 23 Mar 2020 18:22:56 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Christoph Hellwig <hch@....de>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct
 device

On Mon, Mar 23, 2020 at 09:07:38PM +0530, Aneesh Kumar K.V wrote:
> 
> This is what I was trying, but considering I am new to DMA subsystem, I
> am not sure I got all the details correct. The idea is to look at the
> cpu addr and see if that can be used in direct map fashion(is
> bus_dma_limit the right restriction here?) if not fallback to dynamic
> IOMMU mapping.

I don't think we can throw all these complications into the dma
mapping code.  At some point I also wonder what the point is,
especially for scatterlist mappings, where the iommu can coalesce.