[PATCH net-next v1 2/3] dt-bindings: net: micrel: Add keep-preamble-before-sfd

[Stefan Eichenberger <eichest@gmail.com>]

From: Stefan Eichenberger <stefan.eichenberger@toradex.com>

Add a property to activate a Micrel PHY feature that keeps the preamble
enabled before the SFD (Start Frame Delimiter) is transmitted.

This allows to workaround broken Ethernet controllers as found on the
NXP i.MX8MP. Specifically, errata ERR050694 that states:
ENET_QOS: MAC incorrectly discards the received packets when Preamble
Byte does not precede SFD or SMD.

The bit which disables this feature is not documented in the datasheet
from Micrel, but has been found by NXP and Micrel following this
discussion:
https://community.nxp.com/t5/i-MX-Processors/iMX8MP-eqos-not-working-for-10base-t/m-p/2151032

It has been tested on Verdin iMX8MP from Toradex by forcing the PHY to
10MBit. Withouth this property set, no packets are received. With this
property set, reception works fine.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
---
 Documentation/devicetree/bindings/net/micrel.yaml | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/Documentation/devicetree/bindings/net/micrel.yaml b/Documentation/devicetree/bindings/net/micrel.yaml
index f48e9b9120ca0..6d6136014cd62 100644
--- a/Documentation/devicetree/bindings/net/micrel.yaml
+++ b/Documentation/devicetree/bindings/net/micrel.yaml
@@ -165,6 +165,19 @@ allOf:
             supported clocks:
               - The RMII reference input clock. Used to determine the XI
                 input clock.
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: ethernet-phy-id0022.1640
+    then:
+      properties:
+        micrel,keep-preamble-before-sfd:
+          type: boolean
+          description: |
+            If set, the PHY keeps sending preamble bits before SFD in
+            10BASE-T mode. By default, the PHY removes preamble bits
+            before SFD.
   - if:
       properties:
         compatible:
-- 
2.51.0