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

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Date:	Thu, 8 Aug 2013 11:28:44 -0700
From:	Grant Grundler <grundler@...omium.org>
To:	Tomasz Figa <t.figa@...sung.com>
Cc:	Cho KyongHo <pullip.cho@...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, Joerg Roedel <joro@...tes.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Prathyush <prathyush.k@...sung.com>,
	Rahul Sharma <rahul.sharma@...sung.com>,
	Subash Patel <supash.ramaswamy@...aro.org>,
	Grant Grundler <grundler@...omium.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

Tomasz,

On Thu, Aug 8, 2013 at 6:54 AM, Tomasz Figa <t.figa@...sung.com> wrote:
...
>> +     BUG_ON(lv1ent_section(sent));
>
> Is this condition really a critical one, to the point that the system
> should not continue execution?
>
...
>>       if (lv1ent_page(sent)) {
>> -             if (*pgcnt != NUM_LV2ENTRIES)
>> -                     return -EADDRINUSE;
>> -
>> +             BUG_ON(*pgcnt != NUM_LV2ENTRIES);
>
> Ditto.

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.

thanks,
grant
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