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

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Message-ID: <20190814083842.GB22669@8bytes.org>
Date:   Wed, 14 Aug 2019 10:38:43 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     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 Lu Baolu,

On Tue, Jul 30, 2019 at 12:52:26PM +0800, Lu Baolu wrote:
> * iommu_bounce_map(dev, addr, paddr, size, dir, attrs)
>   - Map a buffer start at DMA address @addr in bounce page
>     manner. For buffer parts that doesn't cross a whole
>     minimal IOMMU page, the bounce page policy is applied.
>     A bounce page mapped by swiotlb will be used as the DMA
>     target in the IOMMU page table. Otherwise, the physical
>     address @paddr is mapped instead.
> 
> * iommu_bounce_unmap(dev, addr, size, dir, attrs)
>   - Unmap the buffer mapped with iommu_bounce_map(). The bounce
>     page will be torn down after the bounced data get synced.
> 
> * iommu_bounce_sync(dev, addr, size, dir, target)
>   - Synce the bounced data in case the bounce mapped buffer is
>     reused.

I don't really get why this API extension is needed for your use-case.
Can't this just be done using iommu_map/unmap operations? Can you please
elaborate a bit why these functions are needed?


Regards,

	Joerg