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

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Message-ID: <95395244-bdec-84d2-b81b-3040c076fe4d@ozlabs.ru>
Date:   Tue, 7 Apr 2020 20:12:49 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Christoph Hellwig <hch@....de>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        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, Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct
 device



On 07/04/2020 03:17, Christoph Hellwig wrote:
> On Mon, Apr 06, 2020 at 11:25:09PM +1000, Alexey Kardashevskiy wrote:
>>>> Do you see any serious problem with this approach? Thanks!
>>>
>>> Do you have a link to the whole branch?  The github UI is unfortunately
>>> unusable for that (or I'm missing something).
>>
>> The UI shows the branch but since I rebased and forcepushed it, it does
>> not. Here is the current one with:
>>
>> https://github.com/aik/linux/commits/dma-bypass.3
> 
> Ok, so we use the core bypass without persistent memory, and then
> have another bypass mode on top.  Not great, but I can't think
> of anything better.  Note that your checks for the map_sg case
> aren't very efficient - for one it would make sense to calculate
> the limit only once, 

Good points, I'll post revised version when you post your v3 of this.

> but also it would make sense to reuse the
> calculted diecect mapping addresses instead of doing another pass
> later on in the dma-direct code.

Probably but I wonder what kind of hardware we need to see the
difference. I might try, just need to ride to the office to plug the
cable in my 100GBit eth machines :) Thanks,


-- 
Alexey