linux-kernel - Re: [PATCH 1/2] dma-mapping: check dma

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Date:   Wed, 23 Feb 2022 08:28:13 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, cl@...ux.com, 42.hyeyoo@...il.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        vbabka@...e.cz, David.Laight@...lab.com, david@...hat.com,
        herbert@...dor.apana.org.au, davem@...emloft.net,
        linux-crypto@...r.kernel.org, steffen.klassert@...unet.com,
        netdev@...r.kernel.org, hca@...ux.ibm.com, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
        svens@...ux.ibm.com, linux-s390@...r.kernel.org, michael@...le.cc,
        linux-i2c@...r.kernel.org, wsa@...nel.org
Subject: Re: [PATCH 1/2] dma-mapping: check dma_mask for streaming mapping
 allocs

On 02/22/22 at 04:59pm, Christoph Hellwig wrote:
> On Tue, Feb 22, 2022 at 09:41:43PM +0800, Baoquan He wrote:
> > For newly added streaming mapping APIs, the internal core function
> > __dma_alloc_pages() should check dev->dma_mask, but not
> > ev->coherent_dma_mask which is for coherent mapping.
> 
> No, this is wrong.  dev->coherent_dma_mask is and should be used here.

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 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.

> 
> >
> > 
> > Meanwhile, just filter out gfp flags if they are any of
> > __GFP_DMA, __GFP_DMA32 and __GFP_HIGHMEM, but not fail it. This change
> > makes it  consistent with coherent mapping allocs.
> 
> This is wrong as well.  We want to eventually fail dma_alloc_coherent
> for these, too.  It just needs more work.
>