Hi Enguerrand,

Looks you are covering Module 17 & 23 in Errata[1].

[1] https://ww1.microchip.com/downloads/aemDocuments/documents/UNG/ProductDocuments/Errata/KSZ9477S-Errata-DS80000754.pdf

The errata DS80000754 recommends monitoring potential faults in
half-duplex mode for the KSZ9477 family.

half-duplex is not very common so I just added a critical message
when the fault conditions are detected. The switch can be expected
to be unable to communicate anymore in these states and a software
reset of the switch would be required which I did not implement.

Fixes: b987e98e50ab ("dsa: add DSA switch driver for Microchip KSZ9477")
Signed-off-by: Enguerrand de Ribaucourt <enguerrand.de-
ribaucourt@savoirfairelinux.com>

...

+int ksz9477_errata_monitor(struct ksz_device *dev, int port,
+                          u64 tx_late_col)
+{
+       u32 pmavbc;
+       u8 status;
+       u16 pqm;
+       int ret;
+
+       ret = ksz_pread8(dev, port, REG_PORT_STATUS_0, &status);
+       if (ret)
+               return ret;
+       if (!((status & PORT_INTF_SPEED_MASK) == PORT_INTF_SPEED_MASK) &&
+           !(status & PORT_INTF_FULL_DUPLEX)) {
+               dev_warn_once(dev->dev,
+                             "Half-duplex detected on port %d, transmission
halt may occur\n",
+                             port);
+               /* Errata DS80000754 recommends monitoring potential faults
in
+                * half-duplex mode. The switch might not be able to
communicate anymore
+                * in these states.
+                */
+               if (tx_late_col != 0) {
+                       /* Transmission halt with late collisions */
+                       dev_crit_ratelimited(dev->dev,
+                                            "TX late collisions detected,
transmission may be halted on port %d\n",
+                                            port);
+               }

This covers method 1 of Module 18.

Even though MIB read is not high bandwidth (every 30sec) work,
however, still looping over all ports.
Can you tighten condition for Method 2 because it happens
when both half-duplex and VLAN are enabled?

Adding condition such as 
	ret = ksz_read8(dev, REG_SW_LUE_CTRL_0, &status)
	if (status & SW_VLAN_ENABLE) {
	....
	}

+               ret = ksz_pread16(dev, port, REG_PORT_QM_TX_CNT_0__4, &pqm);
+               if (ret)
+                       return ret;
+               ret = ksz_read32(dev, REG_PMAVBC, &pmavbc);
+               if (ret)
+                       return ret;
+               if ((FIELD_GET(PMAVBC_MASK, pmavbc) <= PMAVBC_MIN) ||
+                   (FIELD_GET(PORT_QM_TX_CNT_M, pqm) >= PORT_QM_TX_CNT_MAX))
{
+                       /* Transmission halt with Half-Duplex and VLAN */
+                       dev_crit_ratelimited(dev->dev,
+                                            "resources out of limits,
transmission may be halted\n");
+               }
+       }
+       return ret;
+}
+

Thanks
Woojung