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From:   Christian Lamparter <chunkeey@gmail.com>
To:     Ivan Mikhaylov <ivan@de.ibm.com>
Cc:     "David S . Miller" <davem@davemloft.net>,
        Rob Herring <robh@kernel.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] net/ibm/emac: wrong bit is used for STA control register write
Date:   Mon, 22 Jan 2018 21:29:39 +0100
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On Monday, January 22, 2018 8:01:46 PM CET Ivan Mikhaylov wrote:
> >Something looks wrong here?! The commit message talks about bit 18, 19 and 20.
> >However, 0x0800, 0x1000, 0x2000 and are like bit 11, 12 and 13? Furthermore,
> >what about the EMAC_STACR_STAC_MASK? shouldn't it be 0x1800 now (or delete it
> >since it doesn't look like it's used anywhere?).
> Christian, nope, it's all fine there, it's big endian.
Ok Thanks. I think I found the relevant info on Wikipedia:
<https://en.wikipedia.org/wiki/Bit_numbering#Usage>

"Little-endian CPUs usually employ "LSB 0" bit numbering, however both bit
numbering conventions can be seen in big-endian machines. Some architectures
like SPARC and Motorola 68000 use "LSB 0" bit numbering, while S/390, PowerPC
and PA-RISC use "MSB 0"."

But as far as the kernel (code) is concerned, all architectures (big or
little endian) have the same BIT marco:
<https://elixir.free-electrons.com/linux/v4.15-rc9/source/include/linux/bitops.h>
|#define BIT(nr)			(1UL << (nr))
So if someone tries to #define EMAC_STACR_STAC_WRITE BIT(18) it would be
0x40000 instead. This is where the confusion is coming from. Can you please
at least mention this somewhere that all the bits in the commit message are
in "MSB 0" format? It's confusing enough as it is ;).

> This commit related only
> to write operation, I'll check MASK and see what I can do with that.
Well, the MASK is not used and it now looks odd. So you might as well
delete it. Maybe as well as all the unused EMACX_STACR_STAC_IND_* macros?

Thanks,
Christian