On Thu, Jul 13, 2017 at 02:21:53PM +0100, Russell King - ARM Linux wrote: > My conclusion of the dma_alloc_noncoherent() and dma_cache_sync() API > when it was introduced is that it's basically a completely broken > interface, and I've never seen any point to it. Maybe some of that is > because it's badly documented - which in turn makes it badly designed > (because there's no specification detailing what it's supposed to be > doing.) > > I'd like to see that thing die... It's not exactly the best interface ever, so any improvement is welcome. I've posted a series to kill dma_alloc_noncoherent in favor of dma_alloc_attrs a short while ago, and a big chunk of it should have made it into 4.12. I plan to kill it off entirely for 4.13. That leaves dma_cache_sync() - it's used by 6 drivers: drivers/net/ethernet/i825xx/lasi_82596.c drivers/net/ethernet/seeq/sgiseeq.c drivers/scsi/53c700.c drivers/scsi/sgiwd93.c drivers/sh/maple/maple.c drivers/tty/serial/mpsc.c Those are used on parisc, mips for a few old SGI systems, the SH dreamcast and powerpc marvell mv64x60 devices. So it shouldn't be too hard to figure out if they could be moved to the normal dma_sync_* calls. On parisc dma_cache_sync is equivalent to dma_sync_single_for_cpu, so that should be fine. On mips the implementation of dma_sync_single_for_cpu is a little more complicated, but both dma_sync_single_for_cpu and dma_cache_sync end up calling __dma_sync_virtual, so they look like the same in the end as well. On SH sync_single_for_device is implemented using dma_cache_sync, and there is no dma_sync_single_for_cpu. On powerpc both dma_sync_single_for_cpu and dma_sync_single_for_device are implemented using the same primitive as dma_cache_sync. In short: killing off dma_cache_sync and using the existing and better defined syncing primitives looks entirely feasible. I'll add it to my TODO list for 4.13.
