On Thu, Jun 06, 2019 at 08:57:49PM +1000, Benjamin Herrenschmidt wrote: > > Wow... that's an odd amount. One thing we could possibly do is add code > > to limit the amount of RAM when we detect that device.... > > Sent too quickly... I mean that *or* force swiotlb at 30-bits on those systems based > on detecting the presence of that device in the device-tree. swiotlb doesn't really help you, as these days swiotlb on matters for the dma_map* case. What would help is a ZONE_DMA that covers these devices. No need to do the 24-bit x86 does, but 30-bit would do it. WIP patch for testing below: diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h index b8286a2013b4..7a367ce87c41 100644 --- a/arch/powerpc/include/asm/page.h +++ b/arch/powerpc/include/asm/page.h @@ -319,6 +319,10 @@ struct vm_area_struct; #endif /* __ASSEMBLY__ */ #include <asm/slice.h> +#if 1 /* XXX: pmac? dynamic discovery? */ +#define ARCH_ZONE_DMA_BITS 30 +#else #define ARCH_ZONE_DMA_BITS 31 +#endif #endif /* _ASM_POWERPC_PAGE_H */ diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c index cba29131bccc..2540d3b2588c 100644 --- a/arch/powerpc/mm/mem.c +++ b/arch/powerpc/mm/mem.c @@ -248,7 +248,8 @@ void __init paging_init(void) (long int)((top_of_ram - total_ram) >> 20)); #ifdef CONFIG_ZONE_DMA - max_zone_pfns[ZONE_DMA] = min(max_low_pfn, 0x7fffffffUL >> PAGE_SHIFT); + max_zone_pfns[ZONE_DMA] = min(max_low_pfn, + ((1UL << ARCH_ZONE_DMA_BITS) - 1) >> PAGE_SHIFT); #endif max_zone_pfns[ZONE_NORMAL] = max_low_pfn; #ifdef CONFIG_HIGHMEM
