linux-kernel - Re: [PATCH] drivers/block/xsysace - replace in(out)_8/in(out)_be16/in(out)

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Date:	Wed, 6 Feb 2013 20:51:26 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	monstr@...str.eu
Cc:	Arnd Bergmann <arnd@...db.de>,
	Grant Likely <grant.likely@...retlab.ca>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	dahinds@...rs.sourceforge.net
Subject: Re: [PATCH] drivers/block/xsysace - replace in(out)_8/in(out)_be16/in(out)_le16
 with generic iowrite(read)8/16(be)

On Wed, Feb 6, 2013 at 6:40 PM, Michal Simek <monstr@...str.eu> wrote:
> One question regarding to asm-generic/io.h about iowrite16be implementation
>
> #define iowrite16be(v, addr)   iowrite16(be16_to_cpu(v), (addr))
> #define iowrite16(v, addr)      writew((v), (addr))
> #define writew(b,addr) __raw_writew(__cpu_to_le16(b),addr)
>
> static inline void __raw_writew(u16 b, volatile void __iomem *addr)
> {
>         *(volatile u16 __force *) addr = b;
> }
>
> How is this suppose to work on Big Endian?
> be16_to_cpu(v) is (v)
> and
> __cpu_to_le16(b) is swab16(v)

Yes.

> On little
> be16_to_cpu(v) is swab16(v)

Yes.

> and
> __cpu_to_le16(swab(b)) is swab16(v)

???

Don't you mean "__cpu_to_le16(b) is (b)"?

> What I would expect is
> #define iowrite16be(v, addr)   __raw_writew(__cpu_to_be16(v), addr)

Indeed, it should be "__cpu_to_be16(v)" instead of "be16_to_cpu(v)".

Same for iowrite32be().

> on Big endian:
> __cpu_to_be16(v) is (v)
> on Little endian:
> __cpu_to_be16(v) is swab(v)
>
> What am I missing here?

But as both conversions are identical (just swapping 2 bytes), It Just Works.

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
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