On Wed, Feb 23, 2022 at 08:28:13AM +0800, Baoquan He wrote:

Could you tell more why this is wrong? According to
Documentation/core-api/dma-api.rst and DMA code, __dma_alloc_pages() is
the core function of dma_alloc_pages()/dma_alloc_noncoherent() which are
obviously streaming mapping,

Why are they "obviously" streaming mappings?

why do we need to check
dev->coherent_dma_mask here? Because dev->coherent_dma_mask is the subset
of dev->dma_mask, it's safer to use dev->coherent_dma_mask in these
places? This is confusing, I talked to Hyeonggon in private mail, he has
the same feeling.

Think of th coherent_dma_mask as dma_alloc_mask.  It is the mask for the
DMA memory allocator.  dma_mask is the mask for the dma_map_* routines.