linux-kernel - Re: [RFC 03/11] net: phy: refactor c45 phy identification sequence

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Date:   Sat, 23 May 2020 21:01:41 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Jeremy Linton <jeremy.linton@....com>, netdev@...r.kernel.org,
        davem@...emloft.net, f.fainelli@...il.com, hkallweit1@...il.com,
        madalin.bucur@....nxp.com, calvin.johnson@....nxp.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 03/11] net: phy: refactor c45 phy identification sequence

On Sat, May 23, 2020 at 09:51:31PM +0200, Andrew Lunn wrote:
> > >  static int get_phy_c45_ids(struct mii_bus *bus, int addr, u32 *phy_id,
> > >  			   struct phy_c45_device_ids *c45_ids) {
> > > -	int phy_reg;
> > > -	int i, reg_addr;
> > > +	int ret;
> > > +	int i;
> > >  	const int num_ids = ARRAY_SIZE(c45_ids->device_ids);
> > >  	u32 *devs = &c45_ids->devices_in_package;
> > 
> > I feel a "reverse christmas tree" complaint brewing... yes, the original
> > code didn't follow it.  Maybe a tidy up while touching this code?
> 
> At minimum, a patch should not make it worse. ret and i should clearly
> be after devs.
> 
> > >  static int get_phy_id(struct mii_bus *bus, int addr, u32 *phy_id,
> > >  		      bool is_c45, struct phy_c45_device_ids *c45_ids)
> > >  {
> > > -	int phy_reg;
> > > +	int ret;
> > >  
> > >  	if (is_c45)
> > >  		return get_phy_c45_ids(bus, addr, phy_id, c45_ids);
> > >  
> > > -	/* Grab the bits from PHYIR1, and put them in the upper half */
> > > -	phy_reg = mdiobus_read(bus, addr, MII_PHYSID1);
> > > -	if (phy_reg < 0) {
> > > +	ret = _get_phy_id(bus, addr, 0, phy_id, false);
> > > +	if (ret < 0) {
> > >  		/* returning -ENODEV doesn't stop bus scanning */
> > > -		return (phy_reg == -EIO || phy_reg == -ENODEV) ? -ENODEV : -EIO;
> > > +		return (ret == -EIO || ret == -ENODEV) ? -ENODEV : -EIO;
> > 
> > Since ret will only ever be -EIO here, this can only ever return
> > -ENODEV, which is a functional change in the code (probably unintended.)
> > Nevertheless, it's likely introducing a bug if the intention is for
> > some other return from mdiobus_read() to be handled differently.
> > 
> > >  	}
> > >  
> > > -	*phy_id = phy_reg << 16;
> > > -
> > > -	/* Grab the bits from PHYIR2, and put them in the lower half */
> > > -	phy_reg = mdiobus_read(bus, addr, MII_PHYSID2);
> > > -	if (phy_reg < 0)
> > > -		return -EIO;
> > 
> > ... whereas this one always returns -EIO on any error.
> > 
> > So, I think you have the potential in this patch to introduce a subtle
> > change of behaviour, which may lead to problems - have you closely
> > analysed why the code was the way it was, and whether your change of
> > behaviour is actually valid?
> 
> I could be remembering this wrongly, but i think this is to do with
> orion_mdio_xsmi_read() returning -ENODEV, not 0xffffffffff, if there
> is no device on the bus at the given address. -EIO is fatal to the
> scan, everything stops with the assumption the bus is broken. -ENODEV
> should not be fatal to the scan.

Maybe orion_mdio_xsmi_read() should be fixed then?  Also, maybe
adding return code documentation for mdiobus_read() / mdiobus_write()
would help MDIO driver authors have some consistency in what
errors they are expected to return (does anyone know for certain?)

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC for 0.8m (est. 1762m) line in suburbia: sync at 13.1Mbps down 424kbps up