Documentation/devicetree/bindings/net/ethernet.txt is confusing because
it says what the MAC should not do, but not what it *should* do:

  * "rgmii-rxid" (RGMII with internal RX delay provided by the PHY, the MAC
     should not add an RX delay in this case)

The gap in semantics is threefold:
1. Is it illegal for the MAC to apply the Rx internal delay by itself,
   and simplify the phy_mode (mask off "rgmii-rxid" into "rgmii") before
   passing it to of_phy_connect? The documentation would suggest yes.
1. For "rgmii-rxid", while the situation with the Rx clock skew is more
   or less clear (needs to be added by the PHY), what should the MAC
   driver do about the Tx delays? Is it an implicit wild card for the
   MAC to apply delays in the Tx direction if it can? What if those were
   already added as serpentine PCB traces, how could that be made more
   obvious through DT bindings so that the MAC doesn't attempt to add
   them twice and again potentially break the link?
3. If the interface is a fixed-link and therefore the PHY object is
   fixed (a purely software entity that obviously cannot add clock
   skew), what is the meaning of the above property?

So an interpretation of the RGMII bindings was chosen that hopefully
does not contradict their intention but also makes them more applied.
The SJA1105 driver understands to act upon "rgmii-*id" phy-mode bindings
if the port is in the PHY role (either explicitly, or if it is a
fixed-link). Otherwise it always passes the duty of setting up delays to
the PHY driver.

The error behavior that this patch adds is required on SJA1105E/T where
the MAC really cannot apply internal delays. If the other end of the
fixed-link cannot apply RGMII delays either (this would be specified
through its own DT bindings), then the situation requires PCB delays.

For SJA1105P/Q/R/S, this is however hardware supported and the error is
thus only temporary. I created a stub function pointer for configuring
delays per-port on RXC and TXC, and will implement it when I have access
to a board with this hardware setup.

Meanwhile do not allow the user to select an invalid configuration.
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>