dt-bindings: net: Correct the documentation of KSZ9021 skew values

The documentation of skew values for the KSZ9021 PHY was misleading
because the driver implementation followed the erroneous information
given in the original KSZ9021 datasheet before it was corrected in
revision 1.2 (Feb 2014). It is probably too late to correct the driver
now because of the many existing device trees, so instead this just
corrects the documentation to explain that what you actually get is not
what you might think when looking at the device tree.

Signed-off-by: James Byrne <james.byrne@origamienergy.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

diff --git a/Documentation/devicetree/bindings/net/micrel-ksz90x1.txt b/Documentation/devicetree/bindings/net/micrel-ksz90x1.txt
index 5100358177c98127c169b046393cffda0f23dec6..b921731cd970e35a9221cae0f33fb982209949df 100644 (file)
--- a/Documentation/devicetree/bindings/net/micrel-ksz90x1.txt
+++ b/Documentation/devicetree/bindings/net/micrel-ksz90x1.txt
@@ -12,8 +12,36 @@ and therefore may overwrite them.
 KSZ9021:
 
   All skew control options are specified in picoseconds. The minimum
-  value is 0, the maximum value is 3000, and it is incremented by 200ps
-  steps.
+  value is 0, the maximum value is 3000, and it can be specified in 200ps
+  steps, *but* these values are in not fact what you get because this chip's
+  skew values actually increase in 120ps steps, starting from -840ps. The
+  incorrect values came from an error in the original KSZ9021 datasheet
+  before it was corrected in revision 1.2 (Feb 2014), but it is too late to
+  change the driver now because of the many existing device trees that have
+  been created using values that go up in increments of 200.
+
+  The following table shows the actual skew delay you will get for each of the
+  possible devicetree values, and the number that will be programmed into the
+  corresponding pad skew register:
+
+  Device Tree Value    Delay   Pad Skew Register Value
+  -----------------------------------------------------
+       0               -840ps          0000
+       200             -720ps          0001
+       400             -600ps          0010
+       600             -480ps          0011
+       800             -360ps          0100
+       1000            -240ps          0101
+       1200            -120ps          0110
+       1400               0ps          0111
+       1600             120ps          1000
+       1800             240ps          1001
+       2000             360ps          1010
+       2200             480ps          1011
+       2400             600ps          1100
+       2600             720ps          1101
+       2800             840ps          1110
+       3000             960ps          1111
 
   Optional properties: