Re: [PATCH 5/6] net: fec: use dma_alloc_noncoherent for m532x

Geert Uytterhoeven
2023-10-17T08:31:46Z

On Mon, Oct 16, 2023 at 11:12 AM Geert Uytterhoeven
<geert@linux-m68k.org> wrote:
> On Tue, Oct 10, 2023 at 4:45 PM Greg Ungerer <gerg@linux-m68k.org> wrote:
> > On 9/10/23 22:58, Christoph Hellwig wrote:
> > > On Mon, Oct 09, 2023 at 11:29:12AM +0100, Robin Murphy wrote:
> > >> It looks a bit odd that this ends up applying to all of Coldfire, while the
> > >> associated cache flush only applies to the M532x platform, which implies
> > >> that we'd now be relying on the non-coherent allocation actually being
> > >> coherent on other Coldfire platforms.
> > >>
> > >> Would it work to do something like this to make sure dma-direct does the
> > >> right thing on such platforms (which presumably don't have caches?), and
> > >> then reduce the scope of this FEC hack accordingly, to clean things up even
> > >> better?
> > >
> > > Probably. Actually Greg comment something along the lines last
> > > time, and mentioned something about just instruction vs instruction
> > > and data cache.
> >
> > I just elaborated on that point a little in response to Robin's email.
> >
> > >>
> > >> diff --git a/arch/m68k/Kconfig.cpu b/arch/m68k/Kconfig.cpu
> > >> index b826e9c677b2..1851fa3fe077 100644
> > >> --- a/arch/m68k/Kconfig.cpu
> > >> +++ b/arch/m68k/Kconfig.cpu
> > >> @@ -27,6 +27,7 @@ config COLDFIRE
> > >> select CPU_HAS_NO_BITFIELDS
> > >> select CPU_HAS_NO_CAS
> > >> select CPU_HAS_NO_MULDIV64
> > >> + select DMA_DEFAULT_COHERENT if !MMU && !M523x
> > >
> > > Although it would probably make more sense to simply not select
> > > CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE and
> > > CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU for these platforms and not
> > > build the non-coherent code at all. This should also include
> > > all coldfire platforms with mmu (M54xx/M548x/M5441x). Then
> > > again for many of the coldfire platforms the Kconfig allows
> > > to select CACHE_WRITETHRU/CACHE_COPYBACK which looks related.
> > >
> > > Greg, any chance you could help out with the caching modes on
> > > coldfire and legacy m68knommu?
> >
> > Sure, yep. I am not aware that the legacy 68000 or 68328 had any caches
> > at all.
>
> 68000 (and derivatives like 68*328) does not have any cache.
> 68360 (which is no longer supported by Linux) is based on CPU32, and
> does not seem to have any caches, although the documentation does
> mention the use of "external cache memories".

As M68K selects NO_DMA if !MMU && !COLDFIRE anyway, this doesn't
matter for legacy^Wclassic m68knommu.

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