linux-kernel - Re: [PATCH 1/4] dma-mapping: Add bounced DMA ops

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Date:   Tue, 14 Jul 2020 13:01:41 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Claire Chang <tientzu@...omium.org>, robh+dt@...nel.org,
        frowand.list@...il.com, hch@....de, m.szyprowski@...sung.com,
        treding@...dia.com, gregkh@...uxfoundation.org,
        saravanak@...gle.com, suzuki.poulose@....com,
        dan.j.williams@...el.com, heikki.krogerus@...ux.intel.com,
        bgolaszewski@...libre.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        tfiga@...omium.org, drinkcat@...omium.org
Subject: Re: [PATCH 1/4] dma-mapping: Add bounced DMA ops

On Mon, Jul 13, 2020 at 12:55:43PM +0100, Robin Murphy wrote:
> On 2020-07-13 10:12, Claire Chang wrote:
>> The bounced DMA ops provide an implementation of DMA ops that bounce
>> streaming DMA in and out of a specially allocated region. Only the
>> operations relevant to streaming DMA are supported.
>
> I think there are too many implicit assumptions here - apparently that 
> coherent allocations will always be intercepted by 
> dma_*_from_dev_coherent(), and that calling into dma-direct won't actually 
> bounce things a second time beyond where you thought they were going, 
> manage coherency for a different address, and make it all go subtly wrong. 
> Consider "swiotlb=force", for instance...
>
> Again, plumbing this straight into dma-direct so that SWIOTLB can simply 
> target a different buffer and always bounce regardless of masks would seem 
> a far better option.

I haven't really had time to read through the details, but I agree that
any bouncing scheme should reuse the swiotlb code and not invent a
parallel infrastructure.