[U-Boot] imx8mq-evk: Outbound network packets lost

[Chris Spencer]

On Sat, 26 Jan 2019 at 01:14, Sergey Kubushyn <ksi at koi8.net> wrote:
> Thanks for a reply. The problem here is not with leftover descriptors -- it
> is MDIO bus not working at all. It is either bogus speed/clock in DM mode or
> something else that I haven't found yet. Reading all zeroes means there is
> no communication with the PHY whatsoever, it comes from bare wire. And there
> is no need for the PHY to be present at all for an MDIO transaction to
> complete successfully -- PHY is a slave device with all clocks coming from
> FEC MDIO so it WILL complete successfully even if it not connected to a PHY.
> It is kinda like SPI that always succeeds for the master.

What I found when debugging the Linux driver is that an MII interrupt
was being delivered too early after the first MDIO read the driver
issues, resulting in it reading back the wrong value. I was able to
reliably stop this from happening by zeroing out ENET_MMFR immediately
before the driver sets ENET_MSCR. If I disable networking in U-Boot
then the problem in the Linux driver doesn't occur at all, so the only
explanation I can come up with is that U-Boot is somehow leaving
something in ENET_MMFR which is being unintentionally triggered when
the Linux driver sets ENET_MSCR. You have a very good point though
because the reads are still completing in U-Boot, even if they always
come back zero, so I'm not really sure how there would end up being
something left over in ENET_MMFR.

Chris