netdev - Re: mii_bus->read return checking in phy

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Date:	Fri, 4 Mar 2011 19:12:10 +0100
From:	Florian Fainelli <florian@...nwrt.org>
To:	"Fleming Andy-AFLEMING" <afleming@...escale.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: mii_bus->read return checking in phy_device.c

On Friday 04 March 2011 19:06:20 Fleming Andy-AFLEMING wrote:
> On Mar 4, 2011, at 11:24, "Florian Fainelli" <florian@...nwrt.org> wrote:
> > Hello Andy,
> > 
> > While debugging a PHY probing issue with the au1000_eth, I stumbled upon
> > this
> > 
> > in drivers/net/phy/phy_device.c:
> >        phy_reg = bus->read(bus, addr, MII_PHYSID1);
> >        
> >        if (phy_reg < 0)
> >        
> >                return -EIO;
> > 
> > most drivers implement phylib's mdio_read callback by simply returning
> > the contents of their MDIO register after a readl, ioread ... which is
> > unsigned. Would not it rather make sense to check for phy_reg <= 0
> > instead?
> 
> That isn't a check for a non-existent PHY.  PHY registers are unsigned
> 16-bit quantities.  The negative 32-bit return value would be the result
> of something going wrong in the bus transaction.

Ok, but 0 is not an acceptable value either for both ID1 and ID2.

> 
> Notice that later the code actually checks to see if the read value was
> mostly 1s...

What if the MDIO bus returns 0 instead of 1? Should that be fixed to return 
0xffff instead in the driver?

> 
> > This can lead for instance to believing that a PHY is present at a wrong
> > address because the MDIO read function returns 0 for that particular
> > register, which is logical because no PHY is present at that address.
> > 
> > I am asking in case I just miss something.
> > 
> > Thank you.
> > --
> > Florian
> 
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