Re: [RFC net-next PATCH 16/16] net: sfp: Add quirk to ignore PHYs

[Sean Anderson]

On 10/5/21 12:45 PM, Sean Anderson wrote:
> On 10/5/21 6:33 AM, Russell King (Oracle) wrote:
> > On Mon, Oct 04, 2021 at 03:15:27PM -0400, Sean Anderson wrote:
> > > Some modules have something at SFP_PHY_ADDR which isn't a PHY. If we try to
> > > probe it, we might attach genphy anyway if addresses 2 and 3 return
> > > something other than all 1s. To avoid this, add a quirk for these modules
> > > so that we do not probe their PHY.
> > >
> > > The particular module in this case is a Finisar SFP-GB-GE-T. This module is
> > > also worked around in xgbe_phy_finisar_phy_quirks() by setting the support
> > > manually. However, I do not believe that it has a PHY in the first place:
> > >
> > > $ i2cdump -y -r 0-31 $BUS 0x56 w
> > >      0,8  1,9  2,a  3,b  4,c  5,d  6,e  7,f
> > > 00: ff01 ff01 ff01 c20c 010c 01c0 0f00 0120
> > > 08: fc48 000e ff78 0000 0000 0000 0000 00f0
> > > 10: 7800 00bc 0000 401c 680c 0300 0000 0000
> > > 18: ff41 0000 0a00 8890 0000 0000 0000 0000
> >
> > Actually, I think that is a PHY. It's byteswapped (which is normal using
> > i2cdump in this way).The real contents of the registers are:
> >
> > 00: 01ff 01ff 01ff 0cc2 0c01 c001 000f 2001
> > 08: 48fc 0e00 78ff 0000 0000 0000 0000 f000
> > 10: 0078 bc00 0000 1c40 0c68 0003 0000 0000
> > 18: 41ff 0000 000a 9088 0000 0000 0000 0000
>
> Ah, thanks for catching this.
>
> > It's advertising pause + asym pause, 1000BASE-T FD, link partner is also
> > advertising 1000BASE-T FD but no pause abilities.
> >
> > When comparing this with a Marvell 88e1111:
> >
> > 00: 1140 7949 0141 0cc2 05e1 0000 0004 2001
> > 08: 0000 0e00 4000 0000 0000 0000 0000 f000
> > 10: 0078 8100 0000 0040 0568 0000 0000 0000
> > 18: 4100 0000 0002 8084 0000 0000 0000 0000
> >
> > It looks remarkably similar. However, The first few reads seem to be
> > corrupted with 0x01ff. It may be that the module is slow to allow the
> > PHY to start responding - we've had similar with Champion One SFPs.
>
> Do you have an an example of how to work around this? Even reading one
> register at a time I still get the bogus 0x01ff. Reading bytewise, a
> reasonable-looking upper byte is returned every other read, but the
> lower byte is 0xff every time.

Ok, upon further experimentation, I can read out something reasonable by using the "consecutive byte" mode

# i2cdump -y -r 0-0x3f 2 0x56 c
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f    0123456789abcdef
00: 01 40 01 6d 01 41 0c c2 0c 01 c5 e1 00 0d 20 01    ?@?m?A??????.? ?
10: 59 f9 0e 00 78 00 00 00 00 00 00 00 00 00 f0 00    Y??.x.........?.
20: 00 78 ac 40 00 00 00 00 0c 68 00 00 00 00 00 00    .x?@....?h......
30: 41 00 00 00 00 0a 90 88 00 00 00 00 00 00 00 00    A....???........

I believe this is just doing i2c_smbus_write_byte+i2c_smbus_read_byte

S Addr Wr [A] Phyaddr [A] P
S Addr Rd [A] [DataHigh] NA P
S Addr Rd [A] [DataLow] NA P

as opposed to i2c_smbus_read_word_data

S Addr Wr [A] Phyaddr [A]
S Addr Rd [A] [DataHigh] A [DataLow] NA P

or i2c_smbus_read_word_data

S Addr Wr [A] Phyaddr [A]
S Addr Rd [A] [DataHigh] NA P
S Addr Wr [A] Phyaddr [A]
S Addr Rd [A] [DataLow] NA P

So now I suppose the question is whether replacing the existing i2c
logic with something like

	i2c_smbus_write_byte(i2c, i2c_mii_phy_addr(phy_id));
	i2c_smbus_read_byte(i2c);
	i2c_smbus_read_byte(i2c);

will break everyone else's SFP phys. If it does, this could be difficult
to do as a quirk because the MII-I2C bus is created before we read the
SFP EEPROM.

--Sean