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Date:   Mon, 28 Mar 2022 01:52:11 +0200
From:   Halil Pasic <pasic@linux.ibm.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Toke =?UTF-8?B?SMO4aWxhbmQtSsO4cmdlbnNlbg==?= <toke@toke.dk>,
        Robin Murphy <robin.murphy@arm.com>,
        Christoph Hellwig <hch@lst.de>,
        Maxime Bizon <mbizon@freebox.fr>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Kalle Valo <kvalo@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Olha Cherevyk <olha.cherevyk@gmail.com>,
        iommu <iommu@lists.linux-foundation.org>,
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        Netdev <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable <stable@vger.kernel.org>,
        Halil Pasic <pasic@linux.ibm.com>
Subject: Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break
 ath9k-based AP
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On Sat, 26 Mar 2022 22:21:03 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Sat, Mar 26, 2022 at 10:06 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Sat, Mar 26, 2022 at 8:49 PM Halil Pasic <pasic@linux.ibm.com> wrote:  
> > >
> > > I agree it CPU modified buffers *concurrently* with DMA can never work,
> > > and I believe the ownership model was conceived to prevent this
> > > situation.  
> >
> > But that just means that the "ownership" model is garbage, and cannot
> > handle this REAL LIFE situation.  
> 
> Just to clarify: I obviously agree that the "both sides modify
> concurrently" obviously cannot work with bounce buffers.
> 
> People still do want to do that, but they'll limit themselves to
> actual cache-coherent DMA when they do so (or do nasty uncached
> accesses but at least no bounce buffering).

Thanks for the explanation!

> 
> But the "bounce ownership back and forth" model comes up empty when
> the CPU wants to read while the DMA is still going on. And that not
> only can work, but *has* worked.
> 
> You could have a new "get me a non-ownership copy" operation of
> course, 

Yes, https://www.spinics.net/lists/linux-wireless/msg222442.html was
mostly about exploring that idea.

> but that hits the problem of "which existing drivers need it?"
> 
> We have no idea, outside of ath9k.
> 
> This is why I believe we have to keep the existing semantics in a way
> that keep ath9k - and any number of unknown other drivers - happy.

I agree.

> 
> And then for the cases where you want to introduce the zeroing because
> you don't know how much data the DMA returned - those are the ones
> you'll have to mark some way.

I have no intention of pursuing this.  When fixing the information leak,
I happened to realize, that a somewhat similar situation can emerge when
mappings are reused. It seemed like an easy fix, so I asked the swiotlb
maintainers, and they agreed. It ain't my field of expertise, and the
drivers I'm interested in don't need this functionality.

Regards,
Halil

> 
>                   Linus