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From:   Jim Quinlan <james.quinlan@broadcom.com>
Date:   Thu, 28 Sep 2023 10:00:23 -0400
Message-ID: <CA+-6iNwj3qUPO99zTCimywyC9sV2aRYw3-TuVQTP8NYWD3LaLg@mail.gmail.com>
Subject: Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted DMA
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Linus Walleij <linus.walleij@linaro.org>,
        Christoph Hellwig <hch@lst.de>,
        bcm-kernel-feedback-list@broadcom.com, jim2101024@gmail.com,
        Russell King <linux@armlinux.org.uk>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Russell King <rmk+kernel@armlinux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Reichel <sebastian.reichel@collabora.com>,
        Mike Rapoport <rppt@kernel.org>,
        Eric DeVolder <eric.devolder@oracle.com>,
        Nathan Chancellor <nathan@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        "moderated list:ARM PORT" <linux-arm-kernel@lists.infradead.org>,
        open list <linux-kernel@vger.kernel.org>,
        Claire Chang <tientzu@chromium.org>
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On Thu, Sep 28, 2023 at 9:32=E2=80=AFAM Arnd Bergmann <arnd@arndb.de> wrote=
:
>
> On Thu, Sep 28, 2023, at 08:07, Jim Quinlan wrote:
> > On Wed, Sep 27, 2023 at 7:10=E2=80=AFPM Linus Walleij <linus.walleij@li=
naro.org> wrote:
> >>
> >> Clearly if you want to do this, surely the ARM-specific
> >> arch/arm/mm/dma-mapping.c and arch/arm/mm/dma-mapping-nommu.c
> >> needs to be removed at the same time?
> >
> >
> > Yes, this is the reason I used "RFC" as the fix looked too easy to be v=
iable :-)
> > I debugged it enough to see that the host driver's
> > writes to the dma_alloc_coherent() region  were not appearing in
> > memory, and that
> > led me to DMA_DIRECT_REMAP.
>
> Usually when you see a mismatch between the data observed by the
> device and the CPU, the problem is an incorrect "dma-coherent"
> property in the DT: either the device is coherent and accesses
> the cache but the CPU tries to bypass it because the property
> is missing, or there is an extraneous property and the CPU
> goes the through the cache but the devices bypasses it.

I just searched, there are no "dt-coherent" properties in our device tree.
Also, even if we did have them, wouldn't things also fail when not using
restricted DMA?

>
> It could also be a driver bug if the device  mixes up the
> address spaces, e.g. passing virt_to_phys(pointer) rather
> than the DMA address returned by dma_alloc_coherent().

This is an Intel 7260 part using the iwlwifi driver, I doubt it has
errors of that kind.

Regards,
Jim Quinlan
Broadcom STB/CM
>
>     Arnd

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