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Commit 20b3f7d7 authored by James Byrne's avatar James Byrne Committed by Jakub Kicinski
Browse files

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: default avatarJames Byrne <james.byrne@origamienergy.com>
Signed-off-by: default avatarJakub Kicinski <jakub.kicinski@netronome.com>
parent cf0eba33

Documentation/devicetree/bindings/net/micrel-ksz90x1.txt

+30 −2
Original line number Diff line number Diff line
@@ -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: