On Tue, Jul 16, 2019 at 5:40 PM Christoph Hellwig <hch@xxxxxx> wrote: > > On Tue, Jul 16, 2019 at 01:02:19PM +0100, Robin Murphy wrote: > >> Lets say 4k video allocation required 300MB cma memory but not required > >> virtual mapping for all the 300MB, its require only 20MB virtually mapped > >> at some specific use case/point of video, and unmap virtual mapping after > >> uses, at that time this functions will be useful, it works like ioremap() > >> for cma_alloc() using dma apis. > > > > Hmm, is there any significant reason that this case couldn't be handled > > with just get_vm_area() plus dma_mmap_attrs(). I know it's only *intended* > > for userspace mappings, but since the basic machinery is there... > > Because the dma helper really are a black box abstraction. > > That being said DMA_ATTR_NO_KERNEL_MAPPING and DMA_ATTR_NON_CONSISTENT > have been a constant pain in the b**t. I've been toying with replacing > them with a dma_alloc_pages or similar abstraction that just returns > a struct page that is guaranteed to be dma addressable by the passed > in device. Then the driver can call dma_map_page / dma_unmap_page / > dma_sync_* on it at well. This would replace DMA_ATTR_NON_CONSISTENT > with a sensible API, and also DMA_ATTR_NO_KERNEL_MAPPING when called > with PageHighmem, while providing an easy to understand API and > something that can easily be fed into the various page based APIs > in the kernel. > > That being said until we get arm moved over the common dma direct > and dma-iommu code, and x86 fully moved over to dma-iommu it just > seems way too much work to even get it into the various architectures > that matter, never mind all the fringe IOMMUs. So for now I've just > been trying to contain the DMA_ATTR_NON_CONSISTENT and > DMA_ATTR_NO_KERNEL_MAPPING in fewer places while also killing bogus > or pointless users of these APIs. I agree with you Christoph, users want page based api, which is useful in many scenarios, but As of now i think we have to move with this type of api which is useful when dma_alloc (for cma )call with DMA_ATTR_NO_KERNEL_MAPPING, and mapped again to kernel space, this api is useful mostly for 32-bit system which has 4GB of limited virtual memory (its very less for android devices) as we have already dma_mmap_attr() for user space mapping. This api is also useful for one who directly want to use cma_alloc() in their own drivers. For example ion-cma.c. Please let me know if any recommendation/suggestion/improvement required ? Regards, Pankaj
