On Wed, Aug 31, 2011 at 3:01 PM, David Daney [off-list ref] wrote:

This patch adds a somewhat generic framework for MDIO bus
multiplexers.  It is modeled on the I2C multiplexer.

The multiplexer is needed if there are multiple PHYs with the same
address connected to the same MDIO bus adepter, or if there is
insufficient electrical drive capability for all the connected PHY
devices.

Conceptually it could look something like this:

                  ------------------
                  | Control Signal |
                  --------+---------
                          |
 ---------------   --------+------
 | MDIO MASTER |---| Multiplexer |
 ---------------   --+-------+----
                    |       |
                    C       C
                    h       h
                    i       i
                    l       l
                    d       d
                    |       |
    ---------       A       B   ---------
    |       |       |       |   |       |
    | PHY@1 +-------+       +---+ PHY@1 |
    |       |       |       |   |       |
    ---------       |       |   ---------
    ---------       |       |   ---------
    |       |       |       |   |       |
    | PHY@2 +-------+       +---+ PHY@2 |
    |       |                   |       |
    ---------                   ---------

This framework configures the bus topology from device tree data.  The
mechanics of switching the multiplexer is left to device specific
drivers.

The follow-on patch contains a multiplexer driven by GPIO lines.

It's amazing how various companies' board designers have come up with
the same brain-dead PHY topologies. We (Freescale) have some similar
code in our tree, but it's not this generically applicable.

Signed-off-by: David Daney <redacted>
Cc: Grant Likely <redacted>
Cc: "David S. Miller" <davem@davemloft.net>

Looks good to me.

Acked-by: Andy Fleming <redacted>