Re: [PATCH v4 2/2] arm: dts: ti: Adds support for AM335x and AM437x

[Andrew Lunn <andrew@lunn.ch>]

> On the AM335x board, the CPSW MDIO and PRUSS MDIO signals are routed to the same physical
> pins (as shown in the schematic, see page 10 “MII_MUX” in tmdxice3359_sch_3h0013_v2_1a.pdf
> from https://www.ti.com/lit/zip/TIDR336 ). Because of this shared routing, the pinmux
> configuration applied by U-Boot for CPSW MDIO remains active even if the CPSW MDIO node is
> later disabled in Linux, and Linux does not automatically revert the pins to their reset state.

It is generally a bad idea to rely on the bootloader. I would make the
CPSW MDIO configure the pins how it needs it. The PRUSS MDIO should
also configure the pins how it needs them. However, it is not as
simple as that...

Looking at the schematic, what you have is ugly. You literally wire
the outputs together, without a hardware mux. For MDC you assume one
is Hi-Z, while the other drives the line. For MDIO it does not matter,
both are inputs. so Hi-Z.

I actually think you might need to represent this in Linux, with
something i would call a pinmux-mux. You give it two sets of pinmux
configurations. The active device claims the mux and gets it to set
the two sets of pinmux as needed. Also, just setting the pinmux to
GPIO is not sufficient, you also need to ensure the GPIO is configured
for input, so the lines go Hi-Z. Often pinmux and GPIO controllers are
interconnected, so the pinmux subsystem might be able to do that for
you.

I don't know if a pinmux-mux already exists in Linux. You probably
want to ask on the pinmux mailing list, or they might have a different
idea how to cleanly do this.

	Andrew