linux-kernel - Re: [PATCH 5.10 51/96] net: dsa: microchip: Fix ksz

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Date:   Wed, 18 Aug 2021 21:26:45 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Ben Hutchings <ben.hutchings@...ensium.com>
Cc:     Pavel Machek <pavel@...x.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.10 51/96] net: dsa: microchip: Fix ksz_read64()

Hi!

> > > [ Upstream commit c34f674c8875235725c3ef86147a627f165d23b4 ]
> > > 
> > > 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.
> > 
> > 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.
> 
> 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.
> 
> > Did this get enough testing for -stable?
> 
> Yes, I actually developed and tested all the ksz8795 changes in 5.10
> before forward-porting to mainline.
> 
> > Is hw little endian or high endian or...?
> 
> The hardware is big-endian and regmap handles any necessary
> byte-swapping for values up to 32 bits.
> 
> > Note that ksz_write64() still contains the strange code, at least in
> > 5.10.
> 
> 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
								
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DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
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