On Sat, Jun 18, 2022 at 3:06 AM Michael Schmitz [off-list ref] wrote:

Am 18.06.2022 um 00:57 schrieb Arnd Bergmann:

quoted

All architecture-independent users of virt_to_bus() and bus_to_virt()
have been fixed to use the dma mapping interfaces or have been
removed now.  This means the definitions on most architectures, and the
CONFIG_VIRT_TO_BUS symbol are now obsolete and can be removed.

The only exceptions to this are a few network and scsi drivers for m68k
Amiga and VME machines and ppc32 Macintosh. These drivers work correctly
with the old interfaces and are probably not worth changing.

The Amiga SCSI drivers are all old WD33C93 ones, and replacing
virt_to_bus by virt_to_phys in the dma_setup() function there would
cause no functional change at all.

Ok, thanks for taking a look here.

drivers/vme/bridges/vme_ca91cx42.c hasn't been used at all on m68k (it
is a PCI-to-VME bridge chipset driver that would be needed on
architectures that natively use a PCI bus). I haven't found anything
that selects that driver, so not sure it is even still in use??

It's gone now, Greg has already taken my patches for this through
the staging tree.

That would allow you to drop the remaining virt_to_bus define from
arch/m68k/include/asm/virtconvert.h.

I could submit a patch to convert the Amiga SCSI drivers to use
virt_to_phys if Geert and the SCSI maintainers think it's worth the churn.

I don't think using virt_to_phys() is an improvement here, as
virt_to_bus() was originally meant as a better abstraction to
replace the use of virt_to_phys() to make drivers portable, before
it got replaced by the dma-mapping interface in turn.

It looks like the Amiga SCSI drivers have an open-coded version of
what dma_map_single() does, to do bounce buffering and cache
management. The ideal solution would be to convert the drivers
actually use the appropriate dma-mapping interfaces and remove
this custom code.

The same could be done for the two vme drivers (scsi/mvme147.c
and ethernet/82596.c), which do the cache management but
apparently don't need swiotlb bounce buffering.

Rewriting the drivers to modern APIs is of course non-trivial,
and if you want a shortcut here, I would suggest introducing
platform specific helpers similar to isa_virt_to_bus() and call
them amiga_virt_to_bus() and vme_virt_to_bus, respectively.

Putting these into a platform specific header file at least helps
clarify that both the helper functions and the drivers using them
are non-portable.

32bit powerpc is a different matter though.

It's similar, but unrelated. The two apple ethernet drivers
(bmac and mace) can again either get changed to use the
dma-mapping interfaces, or get a custom pmac_virt_to_bus()/
pmac_bus_to_virt() helper.

There is also drivers/tty/serial/cpm_uart/cpm_uart_cpm2.c,
which I think just needs a trivial change, but I'm not sure
how to do it correctly.

      Arnd