linux-kernel - Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT

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Date:   Thu, 28 Sep 2023 01:10:27 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Jim Quinlan <james.quinlan@...adcom.com>,
        Christoph Hellwig <hch@....de>
Cc:     bcm-kernel-feedback-list@...adcom.com, jim2101024@...il.com,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Reichel <sebastian.reichel@...labora.com>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Eric DeVolder <eric.devolder@...cle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted DMA

Hi Jim,

thanks for your patch!

On Tue, Sep 26, 2023 at 7:52 PM Jim Quinlan <james.quinlan@...adcom.com> wrote:

> Without this commit, the use of dma_alloc_coherent() while
> using CONFIG_DMA_RESTRICTED_POOL=y 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@...adcom.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