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.