linux-kernel - Re: [PATCH 0/2] Fix incorrect warning from dma-debug

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Date:   Mon, 8 May 2017 11:19:44 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Jon Masters <jcm@...masters.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        iommu@...ts.linux-foundation.org, dmaengine@...r.kernel.org,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] Fix incorrect warning from dma-debug

Hi Jon,

On Sun, May 7, 2017 at 2:06 AM, Jon Masters <jcm@...masters.org> wrote:
> On 05/09/2016 06:00 AM, Robin Murphy wrote:
>> On 09/05/16 10:37, Robin Murphy wrote:
>>> On 08/05/16 11:59, Niklas Söderlund wrote:
>>>> While using CONFIG_DMA_API_DEBUG i came across this warning which I
>>>> think is a false positive. As shown dma_sync_single_for_device() are
>>>> called from the dma_map_single() call path. This triggers the warning
>>>> since the dma-debug code have not yet been made aware of the mapping.
>>>
>>> Almost right ;) The thing being mapped (the SPI device's buffer) and the
>>> thing being synced (the IOMMU's PTE) are entirely unrelated. Due to the
>>> current of_iommu_init() setup, the IOMMU is probed long before
>>> dma_debug_init() gets called, therefore DMA debug is missing entries for
>>> some of the initial page table mappings and gets confused when we update
>>> them later.
>
> What was the eventual followup/fix for this? We're seeing similar traces
> to this during internal testing of 4.11 kernels.

A quick hack is to change arch/arm64/mm/dma-mapping.c:dma_debug_do_init()
from fs_initcall() to arch_initcall().

However, then you loose the debugfs features. A proper fix would involve
splitting dma_debug_init() in two phases: an early one doing the real work,
and a late one registering the debugfs interface.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds