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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Thu, 28 Sep 2023 01:10:27 +0200
Message-ID: <CACRpkdYGPpUUCqmJLT4t+6CNOOmRAh_vYPSPK1SWy+tNycnSqQ@mail.gmail.com>
Subject: Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted DMA
To:     Jim Quinlan <james.quinlan@broadcom.com>,
        Christoph Hellwig <hch@lst.de>
Cc:     bcm-kernel-feedback-list@broadcom.com, jim2101024@gmail.com,
        Russell King <linux@armlinux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        "Russell King (Oracle)" <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 (IBM)" <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>
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Hi Jim,

thanks for your patch!

On Tue, Sep 26, 2023 at 7:52=E2=80=AFPM Jim Quinlan <james.quinlan@broadcom=
.com> wrote:

> Without this commit, the use of dma_alloc_coherent() while
> using CONFIG_DMA_RESTRICTED_POOL=3Dy breaks devices from working.
> For example, the common Wifi 7260 chip (iwlwifi) works fine
> on arm64 with restricted memory but not on arm, unless this
> commit is applied.
>
> Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>

(...)
> +       select DMA_DIRECT_REMAP

Christoph invented that symbol so he can certainly
explain what is missing to use this on ARM.

This looks weird to me, because:
> git grep atomic_pool_init
arch/arm/mm/dma-mapping.c:static int __init atomic_pool_init(void)
kernel/dma/pool.c:static int __init dma_atomic_pool_init(void)

Now you have two atomic DMA pools in the kernel,
and a lot more than that is duplicated. I'm amazed that it
compiles at all.

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?

However I don't think it's that simple, because Christoph would surely
had done this a long time ago if it was that simple.

Yours,
Linus Walleij