linux-kernel - Re: [PATCH 1/2] dma-mapping: fix page attributes for dma_mmap

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Date:   Tue, 6 Aug 2019 07:12:42 +0200
From:   Christoph Hellwig <hch@....de>
To:     Gavin Li <gavinli@...gavinli.com>
Cc:     Christoph Hellwig <hch@....de>, Shawn Anastasio <shawn@...stas.io>,
        Michael Ellerman <mpe@...erman.id.au>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
        linux-mips@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Gavin Li <git@...gavinli.com>
Subject: Re: [PATCH 1/2] dma-mapping: fix page attributes for dma_mmap_*

On Mon, Aug 05, 2019 at 07:55:44PM -0700, Gavin Li wrote:
> >         /* create a coherent mapping */
> >         ret = dma_common_contiguous_remap(page, size, VM_USERMAP,
> > -                       arch_dma_mmap_pgprot(dev, PAGE_KERNEL, attrs),
> > +                       dma_pgprot(dev, PAGE_KERNEL, attrs),
> >                         __builtin_return_address(0));
> >         if (!ret) {
> >                 __dma_direct_free_pages(dev, size, page);
> 
> Is dma_common_contiguous_remap() still necessary in the
> DMA_ATTR_NON_CONSISTENT case? I would presume it would be fine to just
> return a linearly mapped address in that case.

It would not be required for a real DMA_ATTR_NON_CONSISTENT
implementation.  But only parisc and mips actually properly implement
DMA_ATTR_NON_CONSISTENT, everyone ignores it.  Given that the API is
a little ill defined and I have a better replacement in the pipeline
I don't want to start implementing it for other architectures now.