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

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Date:   Thu, 28 Sep 2023 08:07:07 -0400
From:   Jim Quinlan <james.quinlan@...adcom.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Christoph Hellwig <hch@....de>,
        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>,
        Claire Chang <tientzu@...omium.org>
Subject: Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted DMA

On Wed, Sep 27, 2023 at 7:10 PM Linus Walleij <linus.walleij@...aro.org> wrote:
>
> 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.

Hello Linus,

Yes, this is the reason I used "RFC" as the fix looked too easy to be viable :-)
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.

BTW, I tested "restricted-dma" on the master-tip the other day and it
failed for both arm64 and arm.
Please take this with a large grain of salt  as this was a quick test
and  I won't  have time to
confirm and bisect until  next week at the earliest.

Regards,
Jim Quinlan
Broadcom STB/CM


>
> Yours,
> Linus Walleij

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