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

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Date:   Thu, 5 Oct 2023 13:53:33 -0400
From:   Jim Quinlan <james.quinlan@...adcom.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        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

On Mon, Oct 2, 2023 at 2:16 AM Christoph Hellwig <hch@....de> wrote:
>
> On Thu, Sep 28, 2023 at 01:10:27AM +0200, Linus Walleij wrote:
> > (...)
> > > +       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.
>
> Yes, DMA_DIRECT_REMAP should only be used for platforms using the
> generic generic remap that plus straight into dma-direct and
> bypasses arch_dma_alloc.
>
> ARM first needs support to directly set the uncached/wc bits on
> the direct mapping for CMA, which should be fairly simple but require
> wide spread testing.
>
> I'd be happy to work with anyone who wants to look into this.
I'd like to look into this and help make it work for ARCH=arm but you
seem to be saying that you also need help from ARM the company?

Thanks,
Jim Quinlan
Broadcom STB/CM

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