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

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Michael,

On Tue, Jun 28, 2022 at 5:26 AM Michael Schmitz <schmitzmic@gmail.com> wrote:
> Am 28.06.2022 um 09:12 schrieb Michael Schmitz:
> > On 27/06/22 20:26, Geert Uytterhoeven wrote:
> >> 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:
> >>>> From: Arnd Bergmann <arnd@arndb.de>
> >>>>
> >>>> 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.
> >> FTR, the sgiwd93 driver use dma_map_single().
> >
> > Thanks! From what I see, it doesn't have to deal with bounce buffers
> > though?
>
> Leaving the bounce buffer handling in place, and taking a few other
> liberties - this is what converting the easiest case (a3000 SCSI) might
> look like. Any obvious mistakes? The mvme147 driver would be very
> similar to handle (after conversion to a platform device).

Thanks, looks reasonable.

> The driver allocates bounce buffers using kmalloc if it hits an
> unaligned data buffer - can such buffers still even happen these days?

No idea.

> If I understand dma_map_single() correctly, the resulting dma handle
> would be equally misaligned?
>
> To allocate a bounce buffer, would it be OK to use dma_alloc_coherent()
> even though AFAIU memory used for DMA buffers generally isn't consistent
> on m68k?
>
> Thinking ahead to the other two Amiga drivers - I wonder whether
> allocating a static bounce buffer or a DMA pool at driver init is likely
> to succeed if the kernel runs from the low 16 MB RAM chunk? It certainly
> won't succeed if the kernel runs from a higher memory address, so the
> present bounce buffer logic around amiga_chip_alloc() might still need
> to be used here.
>
> Leaves the question whether converting the gvp11 and a2091 drivers is
> actually worth it, if bounce buffers still have to be handled explicitly.

A2091 should be straight-forward, as A3000 is basically A2091 on the
motherboard (comparing the two drivers, looks like someone's been
sprinkling mb()s over the A3000 driver).

I don't have any of these SCSI host adapters (not counting the A590
(~A2091) expansion of the old A500, which is not Linux-capable, and
 hasn't been powered on for 20 years).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds