At the time of introduction, in commit bdeced75 ("net: dsa: felix:
Add PCS operations for PHYLINK"), support for the Lynx PCS inside Felix
was relying, for USXGMII support, on the fact that get_phy_device() is
able to parse the Lynx PCS "device-in-package" registers for this C45
MDIO device and identify it correctly.

However, this was actually working somewhat by mistake (in the sense
that, even though it was detected, it was detected for the wrong
reasons).

The get_phy_c45_ids() function works by iterating through all MMDs
starting from 1 (MDIO_MMD_PMAPMD) and stops at the first one which
returns a non-zero value in the "device-in-package" register pair,
proceeding to see what that non-zero value is.

For the Felix PCS, the first MMD (1, for the PMA/PMD) returns a non-zero
value of 0xffffffff in the "device-in-package" registers. There is a
code branch which is supposed to treat this case and flag it as wrong,
and normally, this would have caught my attention when adding initial
support for this PCS:

	if ((devs_in_pkg & 0x1fffffff) == 0x1fffffff) {
		/* If mostly Fs, there is no device there, then let's probe
		 * MMD 0, as some 10G PHYs have zero Devices In package,
		 * e.g. Cortina CS4315/CS4340 PHY.
		 */

However, this code never actually kicked in, it seems, because this
snippet from get_phy_c45_devs_in_pkg() was basically sabotaging itself,
by returning 0xfffffffe instead of 0xffffffff:

	/* Bit 0 doesn't represent a device, it indicates c22 regs presence */
	*devices_in_package &= ~BIT(0);

Then the rest of the code just carried on thinking "ok, MMD 1 (PMA/PMD)
says that there are 31 devices in that package, each having a device id
of ffff:ffff, that's perfectly fine, let's go ahead and probe this PHY
device".

But after cleanup commit 320ed3bf ("net: phy: split
devices_in_package"), this got "fixed", and now devs_in_pkg is no longer
0xfffffffe, but 0xffffffff. So now, get_phy_device is returning -ENODEV
for the Lynx PCS, because the semantics have remained mostly unchanged:
the loop stops at the first MMD that returns a non-zero value, and that
is MMD 1.

But the Lynx PCS is simply a clause 37 PCS which implements the required
MAC-side functionality for USXGMII (when operated in C45 mode, which is
where C45 devices-in-package detection is relevant to). Of course it
will fail the PMD/PMA test (MMD 1), since it is not a PHY. But it does
implement detection for MDIO_MMD_PCS (3):

- MDIO_DEVS1=0x008a, MDIO_DEVS2=0x0000,
- MDIO_DEVID1=0x0083, MDIO_DEVID2=0xe400

Let get_phy_c45_ids() continue searching for valid MMDs, and don't
assume that every phy_device has a PMA/PMD MMD implemented.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>