Re: [PATCH v8 01/23] dma-direct: return struct page from dma_direct_alloc_from_pool()

Leon Romanovsky <[email protected]> writes:

> On Tue, Jul 21, 2026 at 07:50:10PM +0530, Aneesh Kumar K.V wrote:
>> Leon Romanovsky <[email protected]> writes:
>> 
>> > On Fri, Jul 17, 2026 at 11:34:19PM +0530, Aneesh Kumar K.V (Arm) wrote:
....
>> >>  static void *dma_direct_alloc_no_mapping(struct device *dev, size_t size,
>> >> @@ -247,8 +246,11 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>> >>  	 * the atomic pools instead if we aren't allowed block.
>> >>  	 */
>> >>  	if ((remap || force_dma_unencrypted(dev)) &&
>> >> -	    dma_direct_use_pool(dev, gfp))
>> >> -		return dma_direct_alloc_from_pool(dev, size, dma_handle, gfp);
>> >> +	    dma_direct_use_pool(dev, gfp)) {
>> >> +		page = dma_direct_alloc_from_pool(dev, size, dma_handle,
>> >> +						  &ret, gfp);
>> >> +		return page ? ret : NULL;
>> >
>> > Sorry for joining the discussion late, but the line above caught my
>> > attention.
>> >
>> > Why do we need both ret and page? We can derive cpu_addr from page and
>> > vice versa. Do we really need the &ret parameter? Or, more generally, do
>> > we really need "struct page *"?
>> >
>> > static struct page *__dma_alloc_from_pool(struct device *dev, size_t size,
>> > 		struct gen_pool *pool, void **cpu_addr,
>> > 		bool (*phys_addr_ok)(struct device *, phys_addr_t, size_t))
>> > {
>> > ...
>> > 	*cpu_addr = (void *)addr;
>> > 	memset(*cpu_addr, 0, size);
>> > 	return pfn_to_page(__phys_to_pfn(phys));
>> > }
>> >
>> > Why
>> >
>> 
>> With CONFIG_DMA_DIRECT_REMAP the cpu_addr can be different from
>> page_address.
>
> Can you please point to the code there it can happen?
> __dma_alloc_from_pool() has direct connection between physical address
> and struct page.
>

dma_direct_alloc -> 	remap = IS_ENABLED(CONFIG_DMA_DIRECT_REMAP);
	if ((remap && dma_direct_use_pool(dev, gfp)) {
		page = dma_direct_alloc_from_pool(dev, size,

..
__dma_alloc_from_pool -> 
	addr = gen_pool_alloc(pool, size);
	if (!addr)


We expand the pool as below..

atomic_pool_expand ->

#ifdef CONFIG_DMA_DIRECT_REMAP
	addr = dma_common_contiguous_remap(page, pool_size,
			pgprot_decrypted(pgprot_dmacoherent(PAGE_KERNEL)),
			__builtin_return_address(0));
	if (!addr)
		goto free_page;
#else
	addr = page_to_virt(page);
#endif


-aneesh