[PATCH][RFC] arm: dma-mapping: Add support for allocating/mapping cached buffers

[Marek Szyprowski]

Hi Laura,

On Friday, July 13, 2012 8:02 PM Laura Abbott wrote:

> There are currently no dma allocation APIs that support cached
> buffers. For some use cases, caching provides a signficiant
> performance boost that beats write-combining regions. Add
> apis to allocate and map a cached DMA region.
> 
> Signed-off-by: Laura Abbott <lauraa at codeaurora.org>

I agree that there is a need for cached contiguous memory blocks. I see that your patch
is based on some older version of CMA/dma-mapping code. In v3.5-rc1 CMA has been merged
to mainline kernel together with DMA-mapping redesign patches, so an attribute approach
can be used instead of adding new functions to the API. My original idea was to utilize
the dma_alloc_nonconsistent() call and DMA_ATTR_NONCONSISTENT for allocating/mapping
cached contiguous buffers, but I didn't have enough time for completing this work. 

The main missing piece is the API for managing cache synchronization on such buffers.
There is a dma_cache_synch() functions but it is broken from the API point of view. To
replace it with something better, some additional work is needed for all drivers which
already use it. Also some work in needed for cleanup dma_alloc_nonconsistent() 
implementations for all the architectures using dma_map_ops approach. All this is on my
TODO list, but I currently I'm really busy with other tasks related to CMA (mainly 
bugfixes for some special use-cases).

> ---
>  arch/arm/include/asm/dma-mapping.h |   21 +++++++++++++++++++++
>  arch/arm/mm/dma-mapping.c          |   21 +++++++++++++++++++++
>  2 files changed, 42 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arm/include/asm/dma-mapping.h b/arch/arm/include/asm/dma-mapping.h
> index dc988ff..1565403 100644
> --- a/arch/arm/include/asm/dma-mapping.h
> +++ b/arch/arm/include/asm/dma-mapping.h
> @@ -239,12 +239,33 @@ int dma_mmap_coherent(struct device *, struct vm_area_struct *,
>  extern void *dma_alloc_writecombine(struct device *, size_t, dma_addr_t *,
>  		gfp_t);
> 
> +/**
> + * dma_alloc_cached - allocate cached memory for DMA
> + * @dev: valid struct device pointer, or NULL for ISA and EISA-like devices
> + * @size: required memory size
> + * @handle: bus-specific DMA address
> + *
> + * Allocate some cached memory for a device for
> + * performing DMA.  This function allocates pages, and will
> + * return the CPU-viewed address, and sets @handle to be the
> + * device-viewed address.
> + */
> +extern void *dma_alloc_cached(struct device *, size_t, dma_addr_t *,
> +		gfp_t);
> +
>  #define dma_free_writecombine(dev,size,cpu_addr,handle) \
>  	dma_free_coherent(dev,size,cpu_addr,handle)
> 
> +#define dma_free_cached(dev,size,cpu_addr,handle) \
> +	dma_free_coherent(dev,size,cpu_addr,handle)
> +
>  int dma_mmap_writecombine(struct device *, struct vm_area_struct *,
>  		void *, dma_addr_t, size_t);
> 
> +
> +int dma_mmap_cached(struct device *, struct vm_area_struct *,
> +		void *, dma_addr_t, size_t);
> +
>  /*
>   * This can be called during boot to increase the size of the consistent
>   * DMA region above it's default value of 2MB. It must be called before the
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index b1911c4..f396ddc 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -633,6 +633,20 @@ dma_alloc_writecombine(struct device *dev, size_t size, dma_addr_t
> *handle, gfp_
>  }
>  EXPORT_SYMBOL(dma_alloc_writecombine);
> 
> +/*
> + * Allocate a cached DMA region
> + */
> +void *
> +dma_alloc_cached(struct device *dev, size_t size, dma_addr_t *handle, gfp_t gfp)
> +{
> +	return __dma_alloc(dev, size, handle, gfp,
> +			   pgprot_kernel,
> +			   __builtin_return_address(0));
> +}
> +EXPORT_SYMBOL(dma_alloc_cached);
> +
> +
> +
>  static int dma_mmap(struct device *dev, struct vm_area_struct *vma,
>  		    void *cpu_addr, dma_addr_t dma_addr, size_t size)
>  {
> @@ -664,6 +678,13 @@ int dma_mmap_writecombine(struct device *dev, struct vm_area_struct *vma,
>  }
>  EXPORT_SYMBOL(dma_mmap_writecombine);
> 
> +int dma_mmap_cached(struct device *dev, struct vm_area_struct *vma,
> +			  void *cpu_addr, dma_addr_t dma_addr, size_t size)
> +{
> +	return dma_mmap(dev, vma, cpu_addr, dma_addr, size);
> +}
> +EXPORT_SYMBOL(dma_mmap_cached);
> +
> 
>  /*
>   * Free a buffer as defined by the above mapping.
> --
> 1.7.8.3

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center