linux-kernel - Re: [PATCH v9 03/16] iommu/exynos: fix page table maintenance

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Message-id: <20130816201721.b93a6e3db960e38b29ca7b1a@samsung.com>
Date:	Fri, 16 Aug 2013 20:17:21 +0900
From:	Cho KyongHo <pullip.cho@...sung.com>
To:	Grant Grundler <grundler@...omium.org>
Cc:	Joerg Roedel <joro@...tes.org>, Tomasz Figa <t.figa@...sung.com>,
	Linux ARM Kernel <linux-arm-kernel@...ts.infradead.org>,
	Linux IOMMU <iommu@...ts.linux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux Samsung SOC <linux-samsung-soc@...r.kernel.org>,
	devicetree@...r.kernel.org, Kukjin Kim <kgene.kim@...sung.com>,
	Prathyush <prathyush.k@...sung.com>,
	Rahul Sharma <rahul.sharma@...sung.com>,
	Subash Patel <supash.ramaswamy@...aro.org>,
	Antonios Motakis <a.motakis@...tualopensystems.com>,
	kvmarm@...ts.cs.columbia.edu,
	Sachin Kamat <sachin.kamat@...aro.org>
Subject: Re: [PATCH v9 03/16] iommu/exynos: fix page table maintenance

On Wed, 14 Aug 2013 13:54:53 -0700, Grant Grundler wrote:
> On Wed, Aug 14, 2013 at 3:49 AM, Joerg Roedel <joro@...tes.org> wrote:
> > On Thu, Aug 08, 2013 at 11:28:44AM -0700, Grant Grundler wrote:
> >> I can't speak to the previous BUG_ON(). I believe the EADDRESSINUSE
> >> failures could be either WARN_ON or BUG_ON.   This condition is
> >> clearly a bug in the generic IOMMU allocator and I think that's why
> >> KyongHo Cho used BUG_ON.
> >>
> >> Handing out duplicate addresses will generally lead to some sort of
> >> data corruption or other fault depending on how robust the underlying
> >> device drivers are written.  So my preference is a BUG_ON to
> >> immediately flag this condition instead of hoping a device driver will
> >> correctly handling the dma mapping failure (Some do, most currently
> >> don't).
> >>
> >> WARN_ON() + return -EADDRESSINUSE would be a good alternative.
> >
> > Even if it is a real BUG condition, I don't think it is worth to stop
> > execution at this point. It makes debugging harder and the system less
> > reliable. I prefer to go with the WARN_ON and an error return value.
> 
> I'm ok with WARN_ON and an error return value. This is "valid"
> behavior.  I expect this bug to never happen but if and when it does,
> I want a clear symptom (e.g. WARN_ON) that it happened.
> 

Ok.
Finally, everyone thinks that WARN_ON() is OK.
It will be helpful for the kernel code that uses iommu api.

> My concern is that historically, drivers did not get an error return
> value on failure:
>     ftp://193.166.3.4/pub/linux/kernel/v2.3/patch-html/patch-2.3.47/linux_Documentation_DMA-mapping.txt.html
> 
> or later:
>     https://www.kernel.org/pub/linux/kernel/people/marcelo/linux-2.4/Documentation/DMA-mapping.txt
> 
> And thus, some drivers don't check or attempt to handle mapping
> failures based on this existing code. Here is a recent example:
>      http://comments.gmane.org/gmane.linux.network/272969
> 
> I hope very few or none of those exist since Neil Horman demonstrated
> "dma debugging" can flag this behavior.
> 
> Just for fun, I'll include this link : (apperently 2003 was a good
> year for DMA talks :)
>      http://ols.fedoraproject.org/OLS/Reprints-2003/LinuxSymposium2003-2side.pdf
>      (three talks on DMA issues)
> 

Thank you for the resources :)
Those will be helpful for the guys with me.

> thanks
> grant
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