Subject: Re: [PATCH 1/2] dma-mapping: check dma_mask for streaming mapping allocs

From: Christoph Hellwig <[email-masked]>
Date: Wed, 23 Feb 2022 15:25:55 +0100

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.