net: phy: micrel: Use correct skew values on KSZ9021

Commit ff7bd212cb8a ("net: phy: micrel: fix divisor value for KSZ9031
phy skew") fixed the skew value divisor for the KSZ9031, but left the
code using the same divisor for the KSZ9021, which is incorrect.

The preceding commit c16e69f702b1 ("net: phy: micrel: add documentation
for Micrel KSZ90x1 binding") added the DTS documentation for the
KSZ90x1, changing it from the equivalent file in the Linux kernel to
correctly state that for this part the skew value is set in 120ps steps,
whereas the Linux documentation and driver continue to this day to use
the incorrect value of 200 that came from the original KSZ9021 datasheet
before it was corrected in revision 1.2 (Feb 2014).

This commit sorts out the resulting confusion in a consistent way by
making the following changes:

- Update the documentation to be clear about what the skew values mean,
in the same was as for the KSZ9031.

- Update the Micrel PHY driver to select the appropriate divisor for
both parts.

- Adjust all the device trees that state skew values for KSZ9021 PHYs to
use values based on 120ps steps instead of 200ps steps. This will result
in the same values being programmed into the skew registers as the
equivalent device trees in the Linux kernel do, where it incorrectly
uses 200ps steps (since that's where all these device trees were copied
from).

Signed-off-by: James Byrne <james.byrne@origamienergy.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/doc/device-tree-bindings/net/micrel-ksz90x1.txt b/doc/device-tree-bindings/net/micrel-ksz90x1.txt
index 307f53f726..a214d35fc9 100644
--- a/doc/device-tree-bindings/net/micrel-ksz90x1.txt
+++ b/doc/device-tree-bindings/net/micrel-ksz90x1.txt
@@ -14,6 +14,33 @@ KSZ9021:
   value is 0, the maximum value is 1800, and it is incremented by 120ps
   steps.
 
+  The KSZ9021 hardware supports a range of skew values from negative to
+  positive, where the specific range is property dependent. All values
+  specified in the devicetree are offset by the minimum value so they
+  can be represented as positive integers in the devicetree since it's
+  difficult to represent a negative number in the devictree.
+
+  The following 4-bit values table applies to all the skew properties:
+
+  Pad Skew Value	Delay (ps)	Devicetree Value
+  ------------------------------------------------------
+  0000			-840ps		0
+  0001			-720ps		120
+  0010			-600ps		240
+  0011			-480ps		360
+  0100			-360ps		480
+  0101			-240ps		600
+  0110			-120ps		720
+  0111			0ps		840
+  1000			120ps		960
+  1001			240ps		1080
+  1010			360ps		1200
+  1011			480ps		1320
+  1100			600ps		1440
+  1101			720ps		1560
+  1110			840ps		1680
+  1111			960ps		1800
+
   Optional properties:
 
     - rxc-skew-ps : Skew control of RXC pad