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@xxxxxxxxxxxxxxxxx> --- .../bindings/net/micrel-ksz90x1.txt | 32 +++++++++++++++++-- 1 file changed, 30 insertions(+), 2 deletions(-) diff --git a/Documentation/devicetree/bindings/net/micrel-ksz90x1.txt b/Documentation/devicetree/bindings/net/micrel-ksz90x1.txt index 5100358177c9..b921731cd970 100644 --- 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: -- 2.17.1
