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

[Robin Murphy]

On 28/09/2023 4:16 pm, Arnd Bergmann wrote:
> On Thu, Sep 28, 2023, at 10:00, Jim Quinlan wrote:
> > On Thu, Sep 28, 2023 at 9:32 AM Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxxxx> 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?
>
> Correct, it should be independent of restricted DMA, but it might
> work by chance that way even if it's still wrong. If your DT
> is marked as non-coherent (note: the property to look for
> is "dma-coherent", not "dt-coherent"), can you check the
> datasheet of the SoC to if that is actually correct?
>
> If the chip is designed to support high-speed devices on
> PCIe, it's likely that the PCIe root complex is either coherent
> with the caches, or can (and should) be configured that way
> for performance reasons.
>
> > > 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.
>
> It's unlikely but not impossible, as the driver has some
> unusual constructs, using a lot of coherent mappings that
> might otherwise be streaming mappings, and relying on
> dma_sync_single_for_device(..., DMA_BIDIRECTIONAL) for other
> data, but without the corresponding dma_sync_single_for_cpu().
> If all the testing happens on x86, this might easily lead
> to a bug that only shows up on non-coherent systems but
> is never seen during testing.

Probably the significant thing about restricted DMA is that it forces 
all streaming DMA to be bounce-buffered. That should expose busted 
synchronisation even more decisively than a lack of coherency. If 
there's no IOMMU, then testing the driver in the absence of restricted 
DMA but with "swiotlb=force" should confirm or disprove that.

Robin.

> If the problem is not the "dma-coherent" property, can you
> double-check if using a different PCIe device works, or narrow
> down which specific buffer you saw get corrupted?
>
>       Arnd
>
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