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

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Date:   Wed, 7 Aug 2019 08:04:32 +0200
From:   Christoph Hellwig <hch@....de>
To:     Shawn Anastasio <shawn@...stas.io>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        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 Tue, Aug 06, 2019 at 09:39:06PM +0200, Shawn Anastasio wrote:
>> -#ifdef CONFIG_ARCH_HAS_DMA_MMAP_PGPROT
>>   pgprot_t arch_dma_mmap_pgprot(struct device *dev, pgprot_t prot,
>>   		unsigned long attrs);
>> -#else
>> -# define arch_dma_mmap_pgprot(dev, prot, attrs)	pgprot_noncached(prot)
>> -#endif
>
> Nit, but maybe the prototype should still be ifdef'd here? It at least
> could prevent a reader from incorrectly thinking that the function is
> always present.

Actually it is typical modern Linux style to just provide a prototype
and then use "if (IS_ENABLED(CONFIG_FOO))" to guard the call(s) to it.

>
> Also, like Will mentioned earlier, the function name isn't entirely
> accurate anymore. I second the suggestion of using something like
> arch_dma_noncoherent_pgprot().

As mentioned I plan to remove arch_dma_mmap_pgprot for 5.4, so I'd
rather avoid churn for the short period of time.

> As for your idea of defining
> pgprot_dmacoherent for all architectures as
>
> #ifndef pgprot_dmacoherent
> #define pgprot_dmacoherent pgprot_noncached
> #endif
>
> I think that the name here is kind of misleading too, since this
> definition will only be used when there is no support for proper
> DMA coherency.

Do you have a suggestion for a better name?  I'm pretty bad at naming,
so just reusing the arm name seemed like a good way to avoid having
to make naming decisions myself.