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References: <002701ce941a$eecebdb0$cc6c3910$@samsung.com>
	<1516548.d7oQuzQS7g@amdc1227>
Date: Thu, 8 Aug 2013 11:28:44 -0700
Message-ID: <CANEJEGu1ponghpfvyUsQX16+TZWUV+0KXJ7AehjXpju2N_H+Ew@mail.gmail.com>
Subject: Re: [PATCH v9 03/16] iommu/exynos: fix page table maintenance
From: Grant Grundler <grundler@chromium.org>
To: Tomasz Figa <t.figa@samsung.com>
Cc: Cho KyongHo <pullip.cho@samsung.com>,
        Linux ARM Kernel <linux-arm-kernel@lists.infradead.org>,
        Linux IOMMU <iommu@lists.linux-foundation.org>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Linux Samsung SOC <linux-samsung-soc@vger.kernel.org>,
        devicetree@vger.kernel.org, Joerg Roedel <joro@8bytes.org>,
        Kukjin Kim <kgene.kim@samsung.com>,
        Prathyush <prathyush.k@samsung.com>,
        Rahul Sharma <rahul.sharma@samsung.com>,
        Subash Patel <supash.ramaswamy@linaro.org>,
        Grant Grundler <grundler@chromium.org>,
        Antonios Motakis <a.motakis@virtualopensystems.com>,
        kvmarm@lists.cs.columbia.edu, Sachin Kamat <sachin.kamat@linaro.org>
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Tomasz,

On Thu, Aug 8, 2013 at 6:54 AM, Tomasz Figa <t.figa@samsung.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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