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

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Message-ID: <ffce1af6-a215-dee8-7b5c-2111f43accfd@ozlabs.ru>
Date:   Tue, 24 Mar 2020 14:05:54 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Christoph Hellwig <hch@....de>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     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 24/03/2020 04:22, Christoph Hellwig wrote:
> 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.

This is for persistent memory which you can DMA to/from but yet it does
not appear in the system as a normal memory and therefore requires
special handling anyway (O_DIRECT or DAX, I do not know the exact
mechanics). All other devices in the system should just run as usual,
i.e. use 1:1 mapping if possible.


-- 
Alexey