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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>
This commit is contained in:
James Byrne 2019-09-13 16:46:35 +00:00 committed by Jakub Kicinski
parent cf0eba3342
commit 20b3f7d700
1 changed files with 30 additions and 2 deletions
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32
Documentation/devicetree/bindings/net/micrel-ksz90x1.txt
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@ -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
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