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

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Date:   Tue, 6 Aug 2019 21:39:06 +0200
From:   Shawn Anastasio <shawn@...stas.io>
To:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org
Cc:     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 8/5/19 10:01 AM, Christoph Hellwig wrote:
> diff --git a/include/linux/dma-noncoherent.h b/include/linux/dma-noncoherent.h
> index 3813211a9aad..9ae5cee543c4 100644
> --- a/include/linux/dma-noncoherent.h
> +++ b/include/linux/dma-noncoherent.h
> @@ -42,13 +42,8 @@ void arch_dma_free(struct device *dev, size_t size, void *cpu_addr,
>   		dma_addr_t dma_addr, unsigned long attrs);
>   long arch_dma_coherent_to_pfn(struct device *dev, void *cpu_addr,
>   		dma_addr_t dma_addr);
> -
> -#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.

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 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.