linux-kernel - Re: [PATCH v6 5/8] iommu: Add bounce page APIs

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Message-ID: <ec1dc4e2-626c-9c12-f17c-b51420fc2e81@linux.intel.com>
Date:   Fri, 16 Aug 2019 10:45:13 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     baolu.lu@...ux.intel.com, David Woodhouse <dwmw2@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christoph Hellwig <hch@....de>, ashok.raj@...el.com,
        jacob.jun.pan@...el.com, alan.cox@...el.com, kevin.tian@...el.com,
        mika.westerberg@...ux.intel.com, Ingo Molnar <mingo@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        pengfei.xu@...el.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Jonathan Corbet <corbet@....net>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Alan Cox <alan@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...el.com>
Subject: Re: [PATCH v6 5/8] iommu: Add bounce page APIs

Hi Joerg,

On 8/15/19 11:48 PM, Joerg Roedel wrote:
> On Thu, Aug 15, 2019 at 02:15:32PM +0800, Lu Baolu wrote:
>> iommu_map/unmap() APIs haven't parameters for dma direction and
>> attributions. These parameters are elementary for DMA APIs. Say,
>> after map, if the dma direction is TO_DEVICE and a bounce buffer is
>> used, we must sync the data from the original dma buffer to the bounce
>> buffer; In the opposite direction, if dma is FROM_DEVICE, before unmap,
>> we need to sync the data from the bounce buffer onto the original
>> buffer.
> 
> The DMA direction from DMA-API maps to the protections in iommu_map():
> 
> 	DMA_FROM_DEVICE:	IOMMU_WRITE
> 	DMA_TO_DEVICE:		IOMMU_READ
> 	DMA_BIDIRECTIONAL	IOMMU_READ | IOMMU_WRITE
> 
> And for the sync DMA-API also has separate functions for either
> direction. So I don't see why these extra functions are needed in the
> IOMMU-API.
>

Okay. I understand that adding these APIs in iommu.c is not a good idea.
And, I also don't think merging the bounce buffer implementation into
iommu_map() is feasible since iommu_map() is not DMA API centric.

The bounce buffer implementation will eventually be part of DMA APIs
defined in dma-iommu.c, but currently those APIs are not ready for x86
use yet. So I will put them in iommu/vt-d driver for this time being and
will move them to dma-iommu.c later.

Does this work for you?

Best regards,
Lu Baolu