The MT7530 switch from the MT7621 SoC has 2 ports which can be set up as
internal: port 5 and 6. Arınç reports that the GMAC1 attached to port 5
receives corrupted frames, unless port 6 (attached to GMAC0) has been
brought up by the driver. This is true regardless of whether port 5 is
used as a user port or as a CPU port (carrying DSA tags).

Offline debugging (blind for me) which began in the linked thread showed
experimentally that the configuration done by the driver for port 6
contains a step which is needed by port 5 as well - the write to
CORE_GSWPLL_GRP2 (note that I've no idea as to what it does, apart from
the comment "Set core clock into 500Mhz"). Prints put by Arınç show that
the reset value of CORE_GSWPLL_GRP2 is RG_GSWPLL_POSDIV_500M(1) |
RG_GSWPLL_FBKDIV_500M(40) (0x128), both on the MCM MT7530 from the
MT7621 SoC, as well as on the standalone MT7530 from MT7623NI Bananapi
BPI-R2. Apparently, port 5 on the standalone MT7530 can work under both
values of the register, while on the MT7621 SoC it cannot.

The call path that triggers the register write is:

mt753x_phylink_mac_config() for port 6
-> mt753x_pad_setup()
   -> mt7530_pad_clk_setup()

so this fully explains the behavior noticed by Arınç, that bringing port
6 up is necessary.

The simplest fix for the problem is to extract the register writes which
are needed for both port 5 and 6 into a common mt7530_pll_setup()
function, which is called at mt7530_setup() time, immediately after
switch reset. We can argue that this mirrors the code layout introduced
in mt7531_setup() by commit 42bc4faf ("net: mt7531: only do PLL once
after the reset"), in that the PLL setup has the exact same positioning,
and further work to consolidate the separate setup() functions is not
hindered.

Testing confirms that:

- the slight reordering of writes to MT7530_P6ECR and to
  CORE_GSWPLL_GRP1 / CORE_GSWPLL_GRP2 introduced by this change does not
  appear to cause problems for the operation of port 6 on MT7621 and on
  MT7623 (where port 5 also always worked)

- packets sent through port 5 are not corrupted anymore, regardless of
  whether port 6 is enabled by phylink or not (or even present in the
  device tree)

My algorithm for determining the Fixes: tag is as follows. Testing shows
that some logic from mt7530_pad_clk_setup() is needed even for port 5.
Prior to commit ca366d6c ("net: dsa: mt7530: Convert to PHYLINK
API"), a call did exist for all phy_is_pseudo_fixed_link() ports - so
port 5 included. That commit replaced it with a temporary "Port 5 is not
supported!" comment, and the following commit 38f790a8 ("net: dsa:
mt7530: Add support for port 5") replaced that comment with a
configuration procedure in mt7530_setup_port5() which was insufficient
for port 5 to work. I'm laying the blame on the patch that claimed
support for port 5, although one would have also needed the change from
commit c3b8e079 ("net: dsa: mt7530: setup core clock even in TRGMII
mode") for the write to be performed completely independently from port
6's configuration.

Thanks go to Arınç for describing the problem, for debugging and for
testing.
Reported-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Link: https://lore.kernel.org/netdev/f297c2c4-6e7c-57ac-2394-f6025d309b9d@arinc9.com/
Fixes: 38f790a8 ("net: dsa: mt7530: Add support for port 5")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230307155411.868573-1-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>