Hello! I have a driver for PCI device which uses CMA framework for allocating big coherent blocks of memory for DMA. Allocation looks like: typedef struct { struct list_head list; uint32_t size8; void *kaddr; dma_addr_t paddr; } dma_region_t; LIST_HEAD(region_list); ...... void* AllocDMA( size_t size ) { dma_region_t *new_region; new_region = kmalloc(sizeof(dma_region_t), GFP_KERNEL); new_region->size8 = size; new_region->kaddr = dma_alloc_coherent( NULL, size, &new_region->paddr, GFP_KERNEL | GFP_DMA32 ); list_add(&new_region->list, ®ion_list); printk("pcidev: cma_alloc paddr %pad kaddr %p size %d\n", &new_region->paddr, new_region->kaddr, new_region->size8); return new_region->kaddr; } It works fine on kernel 3.18.26 in 32bit mode. When I reconfigure same kernel to run in 64bit mode(enabling in 'menuconfig' option '64-bit kernel') and trying to use this driver with it allocation failed with message in dmesg: ... [ 1393.835535] fallback device: swiotlb buffer is full (sz: 8388608 bytes) [ 1393.835579] pcidev: cma_alloc paddr 0xffff880234861220 kaddr (null) size 8388608 ... My kernel command line is: swiotlb=16384 iommu=soft cma=256M Anyone can explain this strange behavior? -- D