[PATCH net-next v4 0/2] net: macb: 1000BASE-X on internal PCS

This series adds support for 1000BASE-X autonegotiation to the Cadence macb
driver when using the MAC-internal PCS. The existing driver code is oriented
toward the PCS being used with an on-board SGMII PHY, so uses Cisco
SGMII-style autonegotiation exclusively and does not anticipate e.g. link
state changes arising from fiber attach/detach events since the SGMII endpoint
is permanently attached in such cases. The first patch changes the driver
to monitor the PCS's link; the second extends the existing SGMII
autonegotiation code to also support 1000BASE-X autonegotiation.

Changelog:
- v1: Original patch
  Link: https://lore.kernel.org/netdev/[email protected]/
- v2: Split into two pieces and clean-up of a few details in anrestart().
  Link: https://lore.kernel.org/netdev/[email protected]/
- v3: Fix mistakes in commit message for patch 1 and improve wording.
  Link: https://lore.kernel.org/netdev/[email protected]/
- v4: Fix return value in macb_pcs_config() to indicate to phylink when the
  autonegotiation advertisement has changed and autonegotiation needs to be
  restarted.

Tested on Microchip PolarFire SoC with a variety of SFPs (copper, fiber, GPON)
attached to the I/O Bank-5 PCS lines.

Thanks to Charles Perry, Andrew Lunn, Paolo Abeni, and Théo Lebrun for helpful
suggestions and discussion on this patch series.