[PATCH net 2/2] net: phylink: use USXGMII control-word format to parse Q-USGMII word

[Russell King (Oracle)]

On Thu, Jun 08, 2023 at 07:53:30PM +0200, Maxime Chevallier wrote:
> Hi Russell,
> 
> On Thu, 8 Jun 2023 17:32:52 +0100
> "Russell King (Oracle)" <linux at armlinux.org.uk> wrote:
> 
> > On Thu, Jun 08, 2023 at 06:34:15PM +0200, Maxime Chevallier wrote:
> > > diff --git a/include/uapi/linux/mdio.h b/include/uapi/linux/mdio.h
> > > index 256b463e47a6..1d20a9082507 100644
> > > --- a/include/uapi/linux/mdio.h
> > > +++ b/include/uapi/linux/mdio.h
> > > @@ -444,4 +444,7 @@ static inline __u16 mdio_phy_id_c45(int prtad, int devad)
> > >  #define MDIO_USXGMII_5000FULL		0x1a00	/* 5000Mbps full-duplex */
> > >  #define MDIO_USXGMII_LINK		0x8000	/* PHY link with copper-side partner */
> > >  
> > > +/* Usgmii control word is based on Usxgmii, masking away 2.5, 5 and 10Gbps */
> > > +#define MDIO_USGMII_SPD_MASK		0x0600  
> > 
> > This isn't correct:
> > 
> > 11:9	Speed: Bit 11, 10, 9:
> > 	1 1 1 to 011 = Reserved
> > 	0 1 0 = 1000 Mbps: 1000BASE-TX, 1000BASE-X
> > 	0 0 1 = 100 Mbps: 100BASE-TX, 100BASE-FX
> > 	0 0 0 = 10 Mbps: 10BASET, 10BASE2, 10BASE5
> > 
> > If we only look at bits 10 and 9, then we're interpreting the reserved
> > combinations as valid as well.
> 
> That's why I rewrote the decoding helper instead of simply masking away
> the extra bit, so that we exclude the 0 1 1 combination ( 10G speed ).

I don't think you've understood my comment properly. Here is what
the code is doing:

+#define MDIO_USGMII_SPD_MASK             0x0600
+       switch (lpa & MDIO_USGMII_SPD_MASK) {
#define MDIO_USXGMII_10                 0x0000  /* 10Mbps */
+       case MDIO_USXGMII_10:
+               state->speed = SPEED_10;
+               break;
#define MDIO_USXGMII_100                0x0200  /* 100Mbps */
+       case MDIO_USXGMII_100:
+               state->speed = SPEED_100;
+               break;
#define MDIO_USXGMII_1000               0x0400  /* 1000Mbps */
+       case MDIO_USXGMII_1000:
+               state->speed = SPEED_1000;
+               break;
+       default:
+               state->link = false;
+               return;
+       }

So, this will decode bits 11:9 as:

000	10Mbps
001	100Mbps
010	1000Mgps
011	link = false
100	10Mbps
101	100Mbps
110	1000Mbps
111	link = false

Whereas, USGMII says the last four are all reserved. Why does this
happen? Because the mask is defined as:

+#define MDIO_USGMII_SPD_MASK             0x0600

which only covers bits 10 and 9, masking off bit 11. However, bit 11
is _still_ part of the field, and if it's set, then it is a "reserved"
speed. We should not be just ignoring bit 11.

I hope that helps to clarify.

-- 
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