Re: [PATCH 01/19] dma-mapping: add a Kconfig symbol to indicated arch_dma_prep_coherent presence

[Robin Murphy <robin.murphy@arm.com>]

On 14/01/2019 09:41, Christoph Hellwig wrote:
> Add a Kconfig symbol that indicates an architecture provides a
> arch_dma_prep_coherent implementation, and provide a stub otherwise.
> 
> This will allow the generic dma-iommu code to it while still allowing
> to be built for cache coherent architectures.

I've been pondering this for a while now, and I still can't really come 
up with a case where arch_dma_prep_coherent() would need to behave 
differently from arch_sync_dma_for_device(..., DMA_BIDIRECTIONAL). I 
wonder if we could just save ourselves this little bit of complexity by 
using that instead...

Robin.

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   arch/arm64/Kconfig              | 1 +
>   arch/csky/Kconfig               | 1 +
>   include/linux/dma-noncoherent.h | 6 ++++++
>   kernel/dma/Kconfig              | 3 +++
>   4 files changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index a4168d366127..ae3f581a9bcc 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -13,6 +13,7 @@ config ARM64
>   	select ARCH_HAS_DEVMEM_IS_ALLOWED
>   	select ARCH_HAS_DMA_COHERENT_TO_PFN
>   	select ARCH_HAS_DMA_MMAP_PGPROT
> +	select ARCH_HAS_DMA_PREP_COHERENT
>   	select ARCH_HAS_ACPI_TABLE_UPGRADE if ACPI
>   	select ARCH_HAS_ELF_RANDOMIZE
>   	select ARCH_HAS_FAST_MULTIPLIER
> diff --git a/arch/csky/Kconfig b/arch/csky/Kconfig
> index 398113c845f5..8b84d4362ff6 100644
> --- a/arch/csky/Kconfig
> +++ b/arch/csky/Kconfig
> @@ -1,5 +1,6 @@
>   config CSKY
>   	def_bool y
> +	select ARCH_HAS_DMA_PREP_COHERENT
>   	select ARCH_HAS_SYNC_DMA_FOR_CPU
>   	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
>   	select ARCH_USE_BUILTIN_BSWAP
> diff --git a/include/linux/dma-noncoherent.h b/include/linux/dma-noncoherent.h
> index 69b36ed31a99..9741767e400f 100644
> --- a/include/linux/dma-noncoherent.h
> +++ b/include/linux/dma-noncoherent.h
> @@ -72,6 +72,12 @@ static inline void arch_sync_dma_for_cpu_all(struct device *dev)
>   }
>   #endif /* CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL */
>   
> +#ifdef CONFIG_ARCH_HAS_DMA_PREP_COHERENT
>   void arch_dma_prep_coherent(struct page *page, size_t size);
> +#else
> +static inline void arch_dma_prep_coherent(struct page *page, size_t size)
> +{
> +}
> +#endif /* CONFIG_ARCH_HAS_DMA_PREP_COHERENT */
>   
>   #endif /* _LINUX_DMA_NONCOHERENT_H */
> diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
> index ca88b867e7fe..541128a32c5d 100644
> --- a/kernel/dma/Kconfig
> +++ b/kernel/dma/Kconfig
> @@ -29,6 +29,9 @@ config ARCH_HAS_SYNC_DMA_FOR_CPU
>   config ARCH_HAS_SYNC_DMA_FOR_CPU_ALL
>   	bool
>   
> +config ARCH_HAS_DMA_PREP_COHERENT
> +	bool
> +
>   config ARCH_HAS_DMA_COHERENT_TO_PFN
>   	bool
>   
>