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

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Date:   Tue, 17 Aug 2021 20:20:40 +0200
From:   Ben Hutchings <ben.hutchings@...ensium.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     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()

On Tue, Aug 17, 2021 at 07:56:30PM +0200, Pavel Machek wrote:
> 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.

Ben.

> 
> Best regards,
> 							Pavel
> 							
> > +++ b/drivers/net/dsa/microchip/ksz_common.h
> > @@ -210,12 +210,8 @@ static inline int ksz_read64(struct ksz_device *dev, u32 reg, u64 *val)
> >  	int ret;
> >  
> >  	ret = regmap_bulk_read(dev->regmap[2], reg, value, 2);
> > -	if (!ret) {
> > -		/* Ick! ToDo: Add 64bit R/W to regmap on 32bit systems */
> > -		value[0] = swab32(value[0]);
> > -		value[1] = swab32(value[1]);
> > -		*val = swab64((u64)*value);
> > -	}
> > +	if (!ret)
> > +		*val = (u64)value[0] << 32 | value[1];
> >  
> >  	return ret;
> >  }
> 
> -- 
> DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany



-- 
Ben Hutchings · Senior Embedded Software Engineer, Essensium-Mind · mind.be