On Mon, Jun 07, 2021 at 11:21:20PM +0800, Tianyu Lan wrote:

quoted

dma_map_single can only be used on page baked memory, and if this is
using page backed memory you wouldn't need to do thee phys_to_virt
tricks.  Can someone explain the mess here in more detail?

Sorry. Could you elaborate the issue? These pages in the pb array are not 
allocated by DMA API and using dma_map_single() here is to map these pages' 
address to bounce buffer physical address.

dma_map_single just calls dma_map_page using virt_to_page.  So this
can't work on addresses not in the kernel linear mapping.