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

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Message-ID: <CAK8P3a1NbjYAGCTq2RUfqEX1OgppEV5mKmBJ0t=JiFF13zSytg@mail.gmail.com>
Date:   Mon, 29 Apr 2019 14:39:47 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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

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

       Arnd