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Date:   Wed, 18 Aug 2021 21:26:45 +0200
From:   Pavel Machek <pavel@denx.de>
To:     Ben Hutchings <ben.hutchings@essensium.com>
Cc:     Pavel Machek <pavel@denx.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        Sasha Levin <sashal@kernel.org>
Subject: Re: [PATCH 5.10 51/96] net: dsa: microchip: Fix ksz_read64()
Message-ID: <20210818192645.GA28932@amd>
References: <20210816125434.948010115@linuxfoundation.org>
 <20210816125436.659359567@linuxfoundation.org>
 <20210817175630.GB30136@amd>
 <20210817182040.GA12678@cephalopod>
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Hi!

> > > [ Upstream commit c34f674c8875235725c3ef86147a627f165d23b4 ]
> > >=20
> > > ksz_read64() currently does some dubious byte-swapping on the two
> > > halves of a 64-bit register, and then only returns the high bits.
> > > Replace this with a straightforward expression.
> >=20
> > The code indeed is very strange, but there are just 2 users, and they
> > will now receive byteswapped values, right? If it worked before, it
> > will be broken.
>=20
> The old code swaps the bytes within each 32-bit word, attempts to
> concatenate them into a 64-bit word, then swaps the bytes within the
> 64-bit word.  There is no need for byte-swapping, only (on little-
> endian platforms) a word-swap, which is what the new code does.
>=20
> > Did this get enough testing for -stable?
>=20
> Yes, I actually developed and tested all the ksz8795 changes in 5.10
> before forward-porting to mainline.
>=20
> > Is hw little endian or high endian or...?
>=20
> The hardware is big-endian and regmap handles any necessary
> byte-swapping for values up to 32 bits.
>=20
> > Note that ksz_write64() still contains the strange code, at least in
> > 5.10.
>=20
> It's unnecessarily complex, but it does work.

Thanks for the explanations and sorry for the noise. Indeed
ksz_write64() is quite obfuscated, but I can't see a problem.

Best regards,
								Pavel
							=09
--=20
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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