[PATCH] arm64: dts: imx8mn-var-som: add 20ms delay to ethernet regulator enable

[Shawn Guo]

On Mon, May 29, 2023 at 03:33:12PM -0400, Hugo Villeneuve wrote:
> From: Hugo Villeneuve <hvilleneuve at dimonoff.com>
> 
> This commit is taken from Variscite linux kernel public git repository.
> Original patch author: Nate Drude <nate.d at variscite.com>
> See: https://github.com/varigit/linux-imx/blob/5.15-2.0.x-imx_var01/drivers/net/ethernet/freescale/fec_main.c#L3993-L4050
> 
> The ethernet phy reset was moved from the fec controller to the
> mdio bus, see for example: 0e825b32c033e1998d0ebaf247f5dab3c340e3bf
> 
> When the fec driver managed the reset, the regulator had time to
> settle during the fec phy reset before calling of_mdiobus_register,
> which probes the mii bus for the phy id to match the correct driver.
> 
> Now that the mdio bus controls the reset, the fec driver no longer has
> any delay between enabling the regulator and calling of_mdiobus_register.
> If the regulator voltage has not settled, the phy id will not be read
> correctly and the generic phy driver will be used.
> 
> The following call tree explains in more detail:
> 
> fec_probe
>   fec_reset_phy                               <- no longer introduces delay after migration to mdio reset
>   fec_enet_mii_init
>     of_mdiobus_register
>       of_mdiobus_register_phy
>         fwnode_mdiobus_register_phy
>           get_phy_device                      <- mii probe for phy id to match driver happens here
>           ...
>           fwnode_mdiobus_phy_device_register
>             phy_device_register
>               mdiobus_register_device
>                 mdio_device_reset             <- mdio reset assert / deassert delay happens here
> 
> Add a 20ms enable delay to the regulator to fix the issue.
> 
> Signed-off-by: Hugo Villeneuve <hvilleneuve at dimonoff.com>

Applied, thanks!