[PATCH 1/3] dt-bindings: dsa: microchip: add KSZ low-loss cable errata properties

[Fidelio Lawson <lawson.fidelio@gmail.com>]

Microchip KSZ87xx switches are affected by the "Module 3: Equalizer fix
for short cables" erratum described in DS80000687C.
The embedded PHY receivers are tuned for long, high-loss cables,
which may cause signal distortion when operated with short or low-loss
cabling such as CAT5e or CAT6. In these cases,
the PHY may fail to establish a link due to internal over-amplification.

Two workarounds are provided by Microchip, each configuring a different
indirect register value to adjust the PHY equalizer settings.

This patch introduces two new device tree properties to enable and
select the appropriate workaround:

  - microchip,low-loss-errata-enable: boolean enabling the feature
  - microchip,low-loss-errata: selects workaround 1 or 2 (default: 1)

These properties allow board designers to opt into the errata fix
according to the targeted cable characteristics of their platform.

Signed-off-by: Fidelio Lawson <fidelio.lawson@exotec.com>
---
 .../devicetree/bindings/net/dsa/microchip,ksz.yaml     | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/Documentation/devicetree/bindings/net/dsa/microchip,ksz.yaml b/Documentation/devicetree/bindings/net/dsa/microchip,ksz.yaml
index 8d4a3a9a33fc..ddfbc36aace6 100644
--- a/Documentation/devicetree/bindings/net/dsa/microchip,ksz.yaml
+++ b/Documentation/devicetree/bindings/net/dsa/microchip,ksz.yaml
@@ -85,6 +85,24 @@ properties:
     enum: [2000, 4000, 8000, 12000, 16000, 20000, 24000, 28000]
     default: 8000
 
+  microchip,low-loss-errata-enable:
+    description:
+      The receiver of the embedded PHYs is tuned by default to support long cable length applications. This was developed
+      using low quality, high loss cables. Because of this, the equalizer in the PHY may amplify high amplitude receiver signals
+      to the point that the signal is distorted internally, preventing a link from being established.
+      A more balanced receiver setting has been found that still functions at the long cable lengths and also supports
+      low loss applications.
+      If present, enables the selected errata workaround. If absent, the
+      workaround is disabled regardless of microchip,cat56-errata value.
+    type: boolean
+
+  microchip,low-loss-errata:
+    description:
+      Selects Microchip KSZ87xx CAT5e/CAT6 short-cable errata workaround.
+      1 = Workaround 1, 2 = Workaround 2. If omitted, workaround 1 is applied.
+    enum: [1, 2]
+    default: 1
+
   interrupts:
     maxItems: 1
 

-- 
2.53.0