Re: [PATCH 06/10] dma-direct: refactor the !coherent checks in dma_direct_alloc

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

On 2021-10-21 10:06, Christoph Hellwig wrote:
> Add a big central !dev_is_dma_coherent(dev) block to deal with as much
> as of the uncached allocation schemes and document the schemes a bit
> better.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>   kernel/dma/direct.c | 58 ++++++++++++++++++++++++++++-----------------
>   1 file changed, 36 insertions(+), 22 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 4ffdb524942a1..d66f37f34ba71 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -196,29 +196,46 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   	    !force_dma_unencrypted(dev) && !is_swiotlb_for_alloc(dev))
>   		return dma_direct_alloc_no_mapping(dev, size, dma_handle, gfp);
>   
> -	if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) &&
> -	    !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
> -	    !IS_ENABLED(CONFIG_DMA_GLOBAL_POOL) &&
> -	    !dev_is_dma_coherent(dev) &&
> -	    !is_swiotlb_for_alloc(dev))
> -		return arch_dma_alloc(dev, size, dma_handle, gfp, attrs);
> +	if (!dev_is_dma_coherent(dev)) {
> +		/*
> +		 * Fallback to the arch handler if it exists.  This should
> +		 * eventually go away.
> +		 */
> +		if (!IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) &&
> +		    !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
> +		    !IS_ENABLED(CONFIG_DMA_GLOBAL_POOL) &&
> +		    !is_swiotlb_for_alloc(dev))
> +			return arch_dma_alloc(dev, size, dma_handle, gfp,
> +					      attrs);
>   
> -	if (IS_ENABLED(CONFIG_DMA_GLOBAL_POOL) &&
> -	    !dev_is_dma_coherent(dev))
> -		return dma_alloc_from_global_coherent(dev, size, dma_handle);
> +		/*
> +		 * If there is a global pool, always allocate from it for
> +		 * non-coherent devices.
> +		 */
> +		if (IS_ENABLED(CONFIG_DMA_GLOBAL_POOL))
> +			return dma_alloc_from_global_coherent(dev, size,
> +					dma_handle);
> +
> +		/*
> +		 * Otherwise remap if the architecture is asking for it.  But
> +		 * given that remapping memory is a blocking operation we'll
> +		 * instead have to dip into the atomic pools.
> +		 */
> +		if (IS_ENABLED(CONFIG_DMA_DIRECT_REMAP)) {
> +			if (!gfpflags_allow_blocking(gfp) &&
> +			    !is_swiotlb_for_alloc(dev))
> +				return dma_direct_alloc_from_pool(dev, size,
> +						dma_handle, gfp);
> +			remap = true;

How about:
		remap = IS_ENABLED(CONFIG_DMA_DIRECT_REMAP);

		if (remap && ...)

for a bit less indentation? FWIW I reckon it's slightly more obvious 
that way round.

Robin.

> +		}
> +	}
>   
>   	/*
> -	 * Remapping or decrypting memory may block. If either is required and
> -	 * we can't block, allocate the memory from the atomic pools.
> -	 * If restricted DMA (i.e., is_swiotlb_for_alloc) is required, one must
> -	 * set up another device coherent pool by shared-dma-pool and use
> -	 * dma_alloc_from_dev_coherent instead.
> +	 * Decrypting memory may block, so allocate the memory from the atomic
> +	 * pools if we can't block.
>   	 */
>   	if (IS_ENABLED(CONFIG_DMA_COHERENT_POOL) &&
> -	    !gfpflags_allow_blocking(gfp) &&
> -	    (force_dma_unencrypted(dev) ||
> -	     (IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) &&
> -	      !dev_is_dma_coherent(dev))) &&
> +	    force_dma_unencrypted(dev) && !gfpflags_allow_blocking(gfp) &&
>   	    !is_swiotlb_for_alloc(dev))
>   		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>   
> @@ -226,10 +243,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   	page = __dma_direct_alloc_pages(dev, size, gfp & ~__GFP_ZERO);
>   	if (!page)
>   		return NULL;
> -
> -	if (!dev_is_dma_coherent(dev) && IS_ENABLED(CONFIG_DMA_DIRECT_REMAP)) {
> -		remap = true;
> -	} else if (PageHighMem(page)) {
> +	if (PageHighMem(page)) {
>   		/*
>   		 * Depending on the cma= arguments and per-arch setup,
>   		 * dma_alloc_contiguous could return highmem pages.
> 
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