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

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Date:   Mon, 2 Oct 2023 08:16:28 +0200
From:   Christoph Hellwig <hch@....de>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Jim Quinlan <james.quinlan@...adcom.com>,
        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>
Subject: Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted
 DMA

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.