[PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted DMA

[Jim Quinlan]

On Thu, Sep 28, 2023 at 9:32 AM Arnd Bergmann <arnd at arndb.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 at linaro.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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