Further reading down the document, I realized the returned DMA address (=bus address) is used for setting the dma descriptors or the device register so that the address value is used for actual DMA. But I'm still confused for cases of using iommu. When the iommu exists for the device, does the dma_alloc function allocates virtual address range (which can be physically dis-contiguous) instead of the (physically contiguous) bus address? I mean, will the handle value below be pointing to virtual address for {this process and the device} pair? (virtual to bus address translation being done by iommu). dma_alloc_coherent(struct device *dev, size_t size, dma_addr_t *handle, gfp_t gfp) Any help will be really appreciated. Thanks! Chan Kim From: ckim@etri.re.kr <ckim@etri.re.kr> Sent: Thursday, July 22, 2021 4:33 PM To: kernelnewbies@kernelnewbies.org Subject: a question about dma_set_mask function Hello, I have a question about DNA (I was reading DMA-API-HOWTO.txt <https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt> ). You know regarding DMA, there is cpu virtual address, cpu physical address and bus address for the device. Bus address is converted to cpu physical address using bridge or bus controller to which the device is attached. And for platform device, bus address is equal to the cpu physical address. The DMA-API-HOWTO.txt <https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt> says
When dma_set_mask() or dma_set_mask_and_coherent() is successful, and
returns zero, the kernel saves away this mask you have provided. The
kernel will use this information later when you make DMA mappings.
So I guess when the driver later requests memory for dma using dma_map_..(ex. dma_map_single, or dma_map_sg) function, the kernel allocates memory and sets up page table and it sets the bus address in the ranges supported by the dma_mask. The virtual address is returned by the function, but where is this 'bus address' kept? I see a pointer to the struct page in the function argument, Is it kept in this struct array? Or is the bus address just the physical address(pfn) in the page table?(I guess so) And when the device has a separate iommu(like arm64's SMMU), I guess the DMA virtual address (assigned for dma by the kernel) for the device is first converted to the bus address by iommu and then the bus address is converted to physical address by bus bridge. Is my understanding correct? It's so confusing.. Any explanation or comment will be deeply appreciated. Thanks! Chan Kim