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

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Date:   Thu, 28 Sep 2023 10:00:23 -0400
From:   Jim Quinlan <james.quinlan@...adcom.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Christoph Hellwig <hch@....de>,
        bcm-kernel-feedback-list@...adcom.com, jim2101024@...il.com,
        Russell King <linux@...linux.org.uk>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Russell King <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 <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 Thu, Sep 28, 2023 at 9:32 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Thu, Sep 28, 2023, at 08:07, Jim Quinlan wrote:
> > On Wed, Sep 27, 2023 at 7:10 PM Linus Walleij <linus.walleij@...aro.org> wrote:
> >>
> >> 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?
> >
> >
> > 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.
>
> Usually when you see a mismatch between the data observed by the
> device and the CPU, the problem is an incorrect "dma-coherent"
> property in the DT: either the device is coherent and accesses
> the cache but the CPU tries to bypass it because the property
> is missing, or there is an extraneous property and the CPU
> goes the through the cache but the devices bypasses it.

I just searched, there are no "dt-coherent" properties in our device tree.
Also, even if we did have them, wouldn't things also fail when not using
restricted DMA?

>
> It could also be a driver bug if the device  mixes up the
> address spaces, e.g. passing virt_to_phys(pointer) rather
> than the DMA address returned by dma_alloc_coherent().

This is an Intel 7260 part using the iwlwifi driver, I doubt it has
errors of that kind.

Regards,
Jim Quinlan
Broadcom STB/CM
>
>     Arnd

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