On Thu, Jan 20, 2022 at 03:03:40PM +0100, Christoph Hellwig wrote: > On Wed, Jan 12, 2022 at 06:37:03PM +0000, Matthew Wilcox wrote: > > But let's go further than that (which only brings us to 32 bytes per > > range). For the systems you care about which use an identity mapping, > > and have sizeof(dma_addr_t) == sizeof(phys_addr_t), we can simply > > point the dma_range pointer to the same memory as the phyr. We just > > have to not free it too early. That gets us down to 16 bytes per range, > > a saving of 33%. > > Even without an IOMMU the dma_addr_t can have offsets vs the actual > physical address. Not on x86 except for a weirdo SOC, but just about > everywhere else. The point is dma_map knows if that is happening or not and giving dma_map the option to just return a pointer to the input memory to re-use as the dma list does optimize important widely used cases. Yes, some weirdo SOC cannot do this optimization, but the weirdo SOC will allocate a new memory and return the adjusted dma_addr_t just fine. Ideally we should not pay a cost for weirdo SOC on sane systems. Jason
