[PATCH net-next v3 0/2] net: phy: microchip: add downshift support for LAN88xx

[Nicolai Buchwitz <nb@tipi-net.de>]

Add standard ETHTOOL_PHY_DOWNSHIFT tunable support for the Microchip
LAN88xx PHY, following the same pattern used by Marvell and other PHY
drivers.

Ethernet cables with faulty or missing pairs (specifically C and D)
can successfully auto-negotiate 1000BASE-T but fail to establish a
stable link. The LAN88xx PHY supports automatic downshift to
100BASE-TX after a configurable number of failed attempts (2-5).

Patch 1 adds the get/set tunable implementation.
Patch 2 enables downshift by default with a count of 2. The setting is
stored in the driver's private data so that user changes via ethtool are
preserved across suspend/resume cycles.

Based on an earlier downstream implementation by Phil Elwell.

Tested on Raspberry Pi 3B+ (LAN7515/LAN88xx).

Changes since v2:
  - Store downshift count in priv struct and restore in config_init to
    preserve user settings across suspend/resume (Russell King)

Changes since v1:
  - Replace switch statements with FIELD_GET()/FIELD_PREP() (Andrew Lunn)
  - Drop unused per-count register defines from microchipphy.h

Nicolai Buchwitz (2):
  net: phy: microchip: add downshift tunable support for LAN88xx
  net: phy: microchip: enable downshift by default on LAN88xx

 drivers/net/phy/microchip.c  | 79 +++++++++++++++++++++++++++++++++++-
 include/linux/microchipphy.h |  5 +++
 2 files changed, 83 insertions(+), 1 deletion(-)

-- 
2.51.0