linux-kernel - Re: [PATCH v3 3/3] net: dsa: mv88e6xxx: Support serdes ports on MV88E6123/6131

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Date:   Tue, 20 Oct 2020 22:34:03 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Chris Packham <Chris.Packham@...iedtelesis.co.nz>
Cc:     "andrew@...n.ch" <andrew@...n.ch>,
        "vivien.didelot@...il.com" <vivien.didelot@...il.com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "olteanv@...il.com" <olteanv@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/3] net: dsa: mv88e6xxx: Support serdes ports on
 MV88E6123/6131

On Tue, Oct 20, 2020 at 09:24:04PM +0000, Chris Packham wrote:
> 
> On 20/10/20 11:18 pm, Russell King - ARM Linux admin wrote:
> > On Tue, Oct 20, 2020 at 04:45:58PM +1300, Chris Packham wrote:
> >> +void mv88e6123_serdes_get_regs(struct mv88e6xxx_chip *chip, int port, void *_p)
> >> +{
> >> +	u16 *p = _p;
> >> +	u16 reg;
> >> +	int i;
> >> +
> >> +	if (mv88e6xxx_serdes_get_lane(chip, port) == 0)
> >> +		return;
> >> +
> >> +	for (i = 0; i < 26; i++) {
> >> +		mv88e6xxx_phy_read(chip, port, i, &reg);
> > Shouldn't this deal with a failed read in some way, rather than just
> > assigning the last or possibly uninitialised value to p[i] ?
> 
> mv88e6390_serdes_get_regs() and mv88e6352_serdes_get_regs() also ignore 
> the error. The generic mv88e6xxx_get_regs() memsets p[] to 0xff so if 
> the serdes_get_regs functions just left it alone we'd return 0xffff 
> which is probably better than repeating the last value although it's 
> still ambiguous because 0xffff is a valid value for plenty of these 
> registers.
> 
> Since it looks like I need to come up with an alternative to patch #1 
> I'll concentrate on that but making the serdes_get_regs() a little more 
> error tolerant is a cleanup I can easily tack on onto this series.

Yep, it looks like they all suffer the same problem. Interestingly,
mv88e6xxx_get_regs() does handle the error by avoiding writing the
register entry (so it gets left as 0xffff.)

Incidentally, that's also the value you'll get when reading from a
PHY that doesn't respond, since the MDIO data line is pulled high
when undriven.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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