Subject: Re: [PATCH kernel v3 1/2] dma: Allow mixing bypass and mapped DMA operation

From: Christoph Hellwig <[email-masked]>
Date: Wed, 28 Oct 2020 18:22:01 +0100

On Wed, Oct 28, 2020 at 06:00:29PM +1100, Alexey Kardashevskiy wrote:
> At the moment we allow bypassing DMA ops only when we can do this for
> the entire RAM. However there are configs with mixed type memory
> where we could still allow bypassing IOMMU in most cases;
> POWERPC with persistent memory is one example.
> 
> This adds an arch hook to determine where bypass can still work and
> we invoke direct DMA API. The following patch checks the bus limit
> on POWERPC to allow or disallow direct mapping.
> 
> This adds a CONFIG_ARCH_HAS_DMA_SET_MASK config option to make arch_xxxx
> hooks no-op by default.
> 
> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> ---
>  kernel/dma/mapping.c | 24 ++++++++++++++++++++----
>  kernel/dma/Kconfig   |  4 ++++
>  2 files changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 51bb8fa8eb89..a0bc9eb876ed 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -137,6 +137,18 @@ static inline bool dma_map_direct(struct device *dev,
>  	return dma_go_direct(dev, *dev->dma_mask, ops);
>  }
>  
> +#ifdef CONFIG_ARCH_HAS_DMA_MAP_DIRECT
> +bool arch_dma_map_page_direct(struct device *dev, phys_addr_t addr);
> +bool arch_dma_unmap_page_direct(struct device *dev, dma_addr_t dma_handle);
> +bool arch_dma_map_sg_direct(struct device *dev, struct scatterlist *sg, int nents);
> +bool arch_dma_unmap_sg_direct(struct device *dev, struct scatterlist *sg, int nents);
> +#else
> +#define arch_dma_map_page_direct(d, a) (0)
> +#define arch_dma_unmap_page_direct(d, a) (0)
> +#define arch_dma_map_sg_direct(d, s, n) (0)
> +#define arch_dma_unmap_sg_direct(d, s, n) (0)
> +#endif

A bunch of overly long lines here.  Except for that this looks ok to me.
If you want me to queue up the series I can just fix it up.