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

[Michael Schmitz]

Hi Geert,

On 28/06/22 19:03, Geert Uytterhoeven wrote:
> > 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.
Thanks, I'll take care of Arnd's comments and post a corrected version 
later.
> > The driver allocates bounce buffers using kmalloc if it hits an
> > unaligned data buffer - can such buffers still even happen these days?
> No idea.
Hmmm - I think I'll stick a WARN_ONCE() in there so we know whether this 
code path is still being used.
> > 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).

Yep, and at least the ones in the dma_setup() function are there for no 
reason (the compiler won't reorder stores around the cache flush calls, 
I hope?).

Just leaves the 24 bit DMA mask there (and likely need for bounce buffers).

> 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).

I wonder whether kullervo has survived - that one was an A3000. Should 
have gone to Adrian a few years ago...

Cheers,

    Michael


> Gr{oetje,eeting}s,
>
>                          Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
>
> 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