ARM_LPAE + VMSPLIT_2G cause swiotlb warning on Raspberry Pi 4

[Russell King (Oracle)]

On Wed, May 17, 2023 at 12:06:35PM +0100, Russell King (Oracle) wrote:
> On Wed, May 17, 2023 at 12:54:30PM +0200, Arnd Bergmann wrote:
> > On Wed, May 17, 2023, at 12:34, Arnd Bergmann wrote:
> > > On Wed, May 17, 2023, at 12:24, Stefan Wahren wrote:
> > >> Am 17.05.23 um 12:06 schrieb Robin Murphy:
> > >>> On 2023-05-17 07:23, Stefan Wahren wrote:
> > >
> > > I think LPAE doesn't affect it, the configurations that Stefan
> > > mentioned were LPAE with VMSPLIT_2G (broken), non-LPAE with
> > > VMSPLIT_3G (works) and LPAE with VMSPLIT_3G (works). I would assume
> > > that non-LPAE with VMSPLIT_2G or VMSPLIT_3G_OPT is also broken.
> > >
> > >> This one?
> > >> [    0.000000] software IO TLB: area num 4.
> > >> [    0.000000] software IO TLB: mapped [mem 
> > >> 0x0000000069e7f000-0x000000006de7f000] (64MB)
> > >
> > > Indeed, that is clearly above the .dma_zone_size value from
> > > the machine descriptor.
> > 
> > It looks like Arm does not set the ARCH_LOW_ADDRESS_LIMIT
> > constant for swiotlb, so it gets the default 4GB maximum.
> > 
> > I think this would work on arm, though it's probably not the
> > correct fix in general, as I think MAX_DMA_ADDRESS is meant
> > to refer to the ISA_DMA_API limit, not the ZONE_DMA limit.:
> 
> MAX_DMA_ADDRESS is a _virtual_ address. What is ARCH_LOW_ADDRESS_LIMIT?
> 
> I can find no reference to this in obvious places (mm/ lib/ kernel/
> drivers/base/ drivers/iommu/)

Found it... it's a physial address. So your patch is almost certainly
wrong, since you're passing a virtual address into a function wanting
a physical address.

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