Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS

[Arnd Bergmann <arnd@kernel.org>]

On Sat, Jun 18, 2022 at 3:06 AM Michael Schmitz <schmitzmic@gmail.com> wrote:
> Am 18.06.2022 um 00:57 schrieb Arnd Bergmann:
> >
> > 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