Subject: Re: [PATCH v4 02/13] dma-direct: use DMA_ATTR_CC_SHARED in alloc/free paths

From: Mostafa Saleh <[email-masked]>
Date: Wed, 13 May 2026 13:58:43 +0000

On Tue, May 12, 2026 at 02:33:57PM +0530, Aneesh Kumar K.V (Arm) wrote:
> Propagate force_dma_unencrypted() into DMA_ATTR_CC_SHARED in the
> dma-direct allocation path and use the attribute to drive the related
> decisions.
> 
> This updates dma_direct_alloc(), dma_direct_free(), and
> dma_direct_alloc_pages() to fold the forced unencrypted case into attrs.
> 
> Signed-off-by: Aneesh Kumar K.V (Arm) <[email-masked]>
> ---
>  kernel/dma/direct.c | 44 ++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 36 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index b958f150718a..0c2e1f8436ce 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -201,16 +201,31 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>  		dma_addr_t *dma_handle, gfp_t gfp, unsigned long attrs)
>  {
>  	bool remap = false, set_uncached = false;
> -	bool mark_mem_decrypt = true;
> +	bool mark_mem_decrypt = false;
>  	struct page *page;
>  	void *ret;
>  
> +	/*
> +	 * DMA_ATTR_CC_SHARED is not a caller-visible dma_alloc_*()
> +	 * attribute. The direct allocator uses it internally after it has
> +	 * decided that the backing pages must be shared/decrypted, so the
> +	 * rest of the allocation path can consistently select DMA addresses,
> +	 * choose compatible pools and restore encryption on free.
> +	 */
> +	if (attrs & DMA_ATTR_CC_SHARED)
> +		return NULL;
> +
> +	if (force_dma_unencrypted(dev)) {
> +		attrs |= DMA_ATTR_CC_SHARED;
> +		mark_mem_decrypt = true;
> +	}
> +
>  	size = PAGE_ALIGN(size);
>  	if (attrs & DMA_ATTR_NO_WARN)
>  		gfp |= __GFP_NOWARN;
>  
> -	if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> -	    !force_dma_unencrypted(dev) && !is_swiotlb_for_alloc(dev))
> +	if (((attrs & (DMA_ATTR_NO_KERNEL_MAPPING | DMA_ATTR_CC_SHARED)) ==
> +	     DMA_ATTR_NO_KERNEL_MAPPING) && !is_swiotlb_for_alloc(dev))
>  		return dma_direct_alloc_no_mapping(dev, size, dma_handle, gfp);
>  
>  	if (!dev_is_dma_coherent(dev)) {
> @@ -244,7 +259,7 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>  	 * Remapping or decrypting memory may block, allocate the memory from
>  	 * the atomic pools instead if we aren't allowed block.
>  	 */
> -	if ((remap || force_dma_unencrypted(dev)) &&
> +	if ((remap || (attrs & DMA_ATTR_CC_SHARED)) &&
>  	    dma_direct_use_pool(dev, gfp))
>  		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>  
> @@ -318,11 +333,20 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>  void dma_direct_free(struct device *dev, size_t size,
>  		void *cpu_addr, dma_addr_t dma_addr, unsigned long attrs)
>  {
> -	bool mark_mem_encrypted = true;
> +	bool mark_mem_encrypted = false;
>  	unsigned int page_order = get_order(size);
>  
> -	if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
> -	    !force_dma_unencrypted(dev) && !is_swiotlb_for_alloc(dev)) {
> +	/*
> +	 * if the device had requested for an unencrypted buffer,
> +	 * convert it to encrypted on free
> +	 */
> +	if (force_dma_unencrypted(dev)) {
> +		attrs |= DMA_ATTR_CC_SHARED;
> +		mark_mem_encrypted = true;
> +	}
> +
> +	if (((attrs & (DMA_ATTR_NO_KERNEL_MAPPING | DMA_ATTR_CC_SHARED)) ==
> +	     DMA_ATTR_NO_KERNEL_MAPPING) && !is_swiotlb_for_alloc(dev)) {
>  		/* cpu_addr is a struct page cookie, not a kernel address */
>  		dma_free_contiguous(dev, cpu_addr, size);
>  		return;
> @@ -365,10 +389,14 @@ void dma_direct_free(struct device *dev, size_t size,
>  struct page *dma_direct_alloc_pages(struct device *dev, size_t size,
>  		dma_addr_t *dma_handle, enum dma_data_direction dir, gfp_t gfp)
>  {
> +	unsigned long attrs = 0;
>  	struct page *page;
>  	void *ret;
>  
> -	if (force_dma_unencrypted(dev) && dma_direct_use_pool(dev, gfp))
> +	if (force_dma_unencrypted(dev))
> +		attrs |= DMA_ATTR_CC_SHARED;
> +
> +	if ((attrs & DMA_ATTR_CC_SHARED) && dma_direct_use_pool(dev, gfp))
>  		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);

What about dma_direct_free_pages()? Nothing inside uses attrs, but
that’s quite similar to dma_direct_alloc_pages()

Also, at this point, shouldn’t this patch also remove
force_dma_unencrypted() calls from dma_set_decrypted() and
dma_set_encrypted()?

Thanks,
Mostafa

>  
>  	if (is_swiotlb_for_alloc(dev)) {
> -- 
> 2.43.0
>