ARM_LPAE + VMSPLIT_2G cause swiotlb warning on Raspberry Pi 4

[Russell King (Oracle)]

On Wed, May 17, 2023 at 04:35:35PM +0200, Stefan Wahren wrote:
> Am 17.05.23 um 12:54 schrieb Arnd Bergmann:
> > 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.:
> > 
> > --- a/include/linux/memblock.h
> > +++ b/include/linux/memblock.h
> > @@ -394,7 +394,7 @@ static inline int memblock_get_region_node(const struct memblock_region *r)
> >   #define MEMBLOCK_LOW_LIMIT 0
> >   #ifndef ARCH_LOW_ADDRESS_LIMIT
> > -#define ARCH_LOW_ADDRESS_LIMIT  0xffffffffUL
> > +#define ARCH_LOW_ADDRESS_LIMIT  MAX_DMA_ADDRESS
> >   #endif
> >   phys_addr_t memblock_phys_alloc_range(phys_addr_t size, phys_addr_t align,
> > 
> > Can you give this a try?
> 
> FWIW: Unfortunately this change didn't had any effect.

As expected, because MAX_DMA_ADDRESS is an integer virtual address, and
ARCH_LOW_ADDRESS_LIMIT wants a physical address.

I think we need to define ARCH_LOW_ADDRESS_LIMIT in
arch/arm/incude/asm/dma.h as:

#ifndef CONFIG_ZONE_DMA
...
#else
...
extern phys_addr_t arm_dma_limit;
#define ARCH_LOW_ADDRESS_LIMIT arm_dma_limit
#endif

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