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

From: Aneesh Kumar K.V <[email-masked]>
Date: Thu, 14 May 2026 10:31:30 +0530

Mostafa Saleh <[email-masked]> writes:

> 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()?
>

The final change have 

void dma_direct_free_pages(struct device *dev, size_t size,
...
{
	/*
	 * if the device had requested for an unencrypted buffer,
	 * convert it to encrypted on free
	 */
	bool mark_mem_encrypted =  force_dma_unencrypted(dev);

That got added by "dma-direct: select DMA address encoding from DMA_ATTR_CC_SHARED "

I'll move that hunk into this patch. The overall goal is 

dma_direct_alloc(.. attrs)
{

	if (force_dma_unencrypted(dev))
		attrs |= DMA_ATTR_CC_SHARED;
}

dma_direct_free(.. attrs)
{
	if (force_dma_unencrypted(dev)) {
		attrs |= DMA_ATTR_CC_SHARED;
		mark_mem_encrypted = true;
	}

	if (swiotlb_find_pool(dev, dma_to_phys(dev, dma_addr)))
		mark_mem_encrypted = false;

}

dma_direct_alloc_pages()
{
	if (force_dma_unencrypted(dev))
		attrs |= DMA_ATTR_CC_SHARED;

}

dma_direct_free_pages()
{
	bool mark_mem_encrypted =  force_dma_unencrypted(dev);

	if (swiotlb_find_pool(dev, page_to_phys(page)))
		mark_mem_encrypted = false;

}

-aneesh