[PATCH] 44x ethernet MDIO access

[Ralph Siemsen]

Ralph Siemsen wrote:
> In a related issue, both the PHY read and write functions use a fixed
> time delay (MDIO_DELAY = 50) presently... it seems I need a longer value
> on my hardware, and having such hardcoded values is trouble anyways... I
> suggest we change the code to poll for completion, with a short delay
> and an upper limit on the number of loops.

I've just checked the hardware, it seems that the MDIO clock is driven
at EPB clock divided by 10.  So on the IBM eval board its 7.6MHz.  Each
MDIO transfer involves moving 64 bits of data.  So that means just the
data transfer (never mind any processing in the PHY) takes 8.4us.

The important part to note is that EPB could vary over a considerable
range, so the time needed for MDIO is potentially longer than the 50us
currently used in the code.

Attached is a patch that "works for me" - turns the long udelay() into a
polling loop with short little delays instead.  This allows MDIO_DELAY
to be increased to a larger value (say 150) without slowing down most
systems.

Also I can be used to measure just how long the transfers actually take.
  If MDIO_DEBUG is enabled then you'll see how many "loops" were
needed... on the Ocotea board i get values around 25.

-Ralph
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