From: Christoph Hellwig > Sent: 23 February 2022 14:26 > > On Wed, Feb 23, 2022 at 08:28:13AM +0800, Baoquan He wrote: > > Could you tell more why this is wrong? According to > > Documentation/core-api/dma-api.rst and DMA code, __dma_alloc_pages() is > > the core function of dma_alloc_pages()/dma_alloc_noncoherent() which are > > obviously streaming mapping, > > Why are they "obviously" streaming mappings? > > > why do we need to check > > dev->coherent_dma_mask here? Because dev->coherent_dma_mask is the subset > > of dev->dma_mask, it's safer to use dev->coherent_dma_mask in these > > places? This is confusing, I talked to Hyeonggon in private mail, he has > > the same feeling. > > Think of th coherent_dma_mask as dma_alloc_mask. It is the mask for the > DMA memory allocator. dma_mask is the mask for the dma_map_* routines. I suspect it is all to allow for things like: - A 64bit system with memory above 4G. - A device that can only generate 32bit addresses. - Some feature of the memory system (or bus bridges) that restricts cache snooping to the low 1G of address space. So dma_alloc_coherent() has to allocate memory below 1G. The dma_map functions have to use bounce-buffers for addresses above 4G. dma_alloc_noncoherent() can allocate anything below 4G and so avoid bounce buffers later on. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
