quoted

+    doc: |
+      Loopback component. Identifies where in the network path the
+      loopback is applied.
+    entries:
+      -
+        name: mac
+        doc: MAC loopback
+      -
+        name: pcs
+        doc: PCS loopback
+      -
+        name: phy
+        doc: PHY loopback
+      -
+        name: module
+        doc: Pluggable module (e.g. CMIS (Q)SFP) loopback

Should we also add "serdes" ?

What is the difference between SERDES and PCS?

Maybe we should also make it clear PHY means Ethernet PHY. The Marvell
Generic PHYs have

phy-mvebu-a3700-comphy.c:#define COMPHY_DIG_LOOPBACK_EN		0x23
phy-mvebu-cp110-comphy.c:#define MVEBU_COMPHY_LOOPBACK(n)		(0x88c + (n) * 0x1000)

which suggests it can also do loopback. We don't want PHY meaning two
different things.

And maybe we should comment that a Base-T SFP module which contains an
Ethernet PHY should appear under PHY, not module? When Linux is
driving the PHY, that should just happen. But if it is firmware
driving the PHY in the module, we should make our expectations of the
firmware clear.

	 Andrew