Hello,
This patch implements the “Module 3: Equalizer fix for short cables” erratum
described in Microchip document DS80000687C for KSZ87xx switches.
According to the erratum, the embedded PHY receiver in KSZ87xx switches is
tuned by default for long, high-loss Ethernet cables. When operating with
short or low-loss cables (for example CAT5e or CAT6), the PHY equalizer may
over-amplify the incoming signal, leading to internal distortion and link
establishment failures.
Microchip provides two workarounds, each requiring a write to a different
indirect PHY register access mechanism.
The workaround requires programming internal PHY/DSP registers located in the
LinkMD table, accessed through the KSZ8 indirect register mechanism. Since these
registers belong to the switch address space and are not directly accessible
from a standalone PHY driver, the erratum control is modeled as a vendor-specific
Clause 22 PHY register, virtualized by the KSZ8 DSA driver.
Reads and writes to this register are intercepted by ksz8_r_phy() /
ksz8_w_phy() and translated into the required TABLE_LINK_MD_V indirect accesses.
The erratum affects the shared PHY analog front-end and therefore applies
globally to the switch.
The control register defines the following modes:
0: disabled (default behavior)
1: EQ training workaround
2: LPF 90 MHz
3: LPF 62 MHz
4: LPF 55 MHz
5: LPF 44 MHz
The register can be read and written from userspace via a phy tunable.
Note that current ethtool userspace only supports a fixed set of PHY tunables;
vendor-specific tunables may require either phytool or a newer userspace extension.
This series is based on Linux v7.0-rc1.
Signed-off-by: Fidelio Lawson <fidelio.lawson@exotec.com>
---
Changes in v3:
- Exposed all LPF bandwidth values supported by the hardware.
- Added phy tunable.
- Link to v2: https://patch.msgid.link/20260408-ksz87xx_errata_low_loss_connections-v2-1-9cfe38691713@exotec.com
Changes in v2:
- Dropped the device tree approach based on review feedback
- Modeled the errata control as a vendor-specific Clause 22 PHY register
- Added KSZ87xx-specific guards and replaced magic values with named macros
- Rebased on Linux v7.0-rc1
- Link to v1: https://patch.msgid.link/20260326-ksz87xx_errata_low_loss_connections-v1-0-79a698f43626@exotec.com
---
Fidelio Lawson (3):
net: dsa: microchip: implement KSZ87xx Module 3 low-loss cable errata
net: ethtool: add KSZ87xx low-loss PHY tunable
net: phy: micrel: expose KSZ87xx low-loss erratum via PHY tunable
drivers/net/dsa/microchip/ksz8.c | 45 ++++++++++++++++++++++++++++++++++
drivers/net/dsa/microchip/ksz8_reg.h | 36 ++++++++++++++++++++++++++-
drivers/net/dsa/microchip/ksz_common.h | 3 +++
drivers/net/phy/micrel.c | 39 +++++++++++++++++++++++++++++
include/uapi/linux/ethtool.h | 1 +
net/ethtool/common.c | 1 +
net/ethtool/ioctl.c | 1 +
7 files changed, 125 insertions(+), 1 deletion(-)
---
base-commit: 2d1373e4246da3b58e1df058374ed6b101804e07
change-id: 20260323-ksz87xx_errata_low_loss_connections-b65e76e2b403
Best regards,
--
Fidelio Lawson <fidelio.lawson@exotec.com>