linux-kernel - Re: [PATCH] m68k: io: Fix io{read,write}{16,32}be() for Coldfire peripherals

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Date:   Fri, 14 Jun 2019 16:49:45 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Greg Ungerer <gerg@...ux-m68k.org>,
        Angelo Dureghello <angelo@...am.it>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] m68k: io: Fix io{read,write}{16,32}be() for Coldfire peripherals

Hi Arnd,

On Mon, Apr 29, 2019 at 2:40 PM Arnd Bergmann <arnd@...db.de> wrote:
> On Mon, Apr 29, 2019 at 10:19 AM Geert Uytterhoeven
> <geert@...ux-m68k.org> wrote:
> > The generic definitions of mmio_{read,write}{16,32}be() in lib/iomap.c
> > assume that the {read,write}[wl]() I/O accessors always use little
> > endian accesses, and swap the result.
> >
> > However, the Coldfire versions of the {read,write}[wl]() I/O accessors are
> > special, in that they use native big endian instead of little endian for
> > accesses to the on-SoC peripheral block, thus violating the assumption.
> >
> > Fix this by providing our own variants, using the raw accessors,
> > reinstating the old behavior.  This is fine on m68k, as no special
> > barriers are needed, and also avoids swapping data twice.
> >
> > Reported-by: Angelo Dureghello <angelo@...am.it>
> > Fixes: aecc787c06f4300f ("iomap: Use non-raw io functions for io{read|write}XXbe")
> > Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
> > ---
> > This can be reverted later, after this oddity of the Coldfire I/O
> > support has been fixed, and drivers have been updated.
> > ---
> >  arch/m68k/include/asm/io.h | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/arch/m68k/include/asm/io.h b/arch/m68k/include/asm/io.h
> > index aabe6420ead2a599..d47e7384681ab1cd 100644
> > --- a/arch/m68k/include/asm/io.h
> > +++ b/arch/m68k/include/asm/io.h
> > @@ -8,6 +8,12 @@
> >  #include <asm/io_mm.h>
> >  #endif
> >
> > +#define mmio_read16be(addr)            __raw_readw(addr)
> > +#define mmio_read32be(addr)            __raw_readl(addr)
> > +
> > +#define mmio_write16be(val, port)      __raw_writew((val), (port))
> > +#define mmio_write32be(val, port)      __raw_writel((val), (port))
> > +
> >  #include <asm-generic/io.h>
>
> This looks correct to me, but there are two points that stick out to me:
>
> - why do you only do this for mmio and not for pio?

Because no one had a need for it? ;-)

Now seriously, m68k only has MMIO, no PIO.  Any PIO, if used, is for ISA or
PCMCIA I/O accesses, which are little endian.

> - why do you even use the generic_iomap wrappers rather than
>   the trivial asm-generic versions of those functions?

Looking at git history, that was done to fix some link errors, which no
longer seem to happen with the more mature asm-generic infrastructure we
have now.

So probably we can drop the selection of GENERIC_IOMAP and inclusion
of <asm-generic/iomap.h>, after fixing a few compiler warnings like:

    include/asm-generic/io.h: In function ‘ioread8_rep’
    arch/m68k/include/asm/io_mm.h:371:44: warning: passing argument 1
of ‘raw_insb’ discard ‘const’ qualifier from pointer target type
[-Wdiscarded-qualifiers]
     #define readsb(port, buf, nr)     raw_insb((port), (u8 *)(buf), (nr))
    arch/m68k/include/asm/raw_io.h:101:50: note: expected ‘volatile u8
*’ {aka ‘volatile unsigned char *’} but argument is of type ‘const
volatile void *’
    static inline void raw_insb(volatile u8 __iomem *port, u8 *buf,
unsigned int len)

and some regression testing, of course.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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