linux-kernel - Re: [PATCH] Revert "regmap-mmio: Use native endianness for read/write"

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Date:	Tue, 26 Jan 2016 14:07:58 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Mark Brown <broonie@...nel.org>, Simon Arlott <simon@...e.lp0.eu>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "regmap-mmio: Use native endianness for read/write"

On Tuesday 26 January 2016 10:24:22 Johannes Berg wrote:
> On Tue, 2016-01-26 at 10:09 +0100, Arnd Bergmann wrote:
> > * On 32-bit architectures, you generally cannot do 64-bit atomic I/O
> >   operations, and we have two implementations that do it
> > nonatomically,
> >   depending on how a device is wired to the bus, see
> >   include/linux/io-64-nonatomic-{hi-lo,lo-hi}.h.
> >   I think we should just not go there for regmap unless we absolutely
> >   have to.
> 
> regmap-mmio doesn't define the 8-bit accesses for 32-bit platforms, so
> this is a non-issue, I think.

Ok, good.

> > There is still one open question about the defaults: I think we all
> > agree that there is no way we can change the default for
> > compatible="syscon" devices on ARM to from little-endian to cpu-
> > endian, as that would break everything. Annotating the MIPS dts files
> > as "cpu-endian" and leaving the rest to default to "little" is
> > probably best here.
> 
> Since regmap-mmio in practice was always little endian, we should
> definitely make that consistent and explicit. Annotating those that
> need special CPU-endian handling (MIPS with the byteswap engine) would
> be best, I agree.
> 
> Note that I made a mistake here yesterday - the *reg* for MMIO is still
> NATIVE, while the *value* is LITTLE_ENDIAN. Looks like regmap-core can
> byteswap both, which makes sense for I2C and similar busses.

I see, that's something I completely missed, and it's logical that
the reg number must be native endian for MMIO but not for everything.

	Arnd