[PATCH net v2 2/7] net: dsa: mt7530: fix trapping frames with multiple CPU ports on MT7530

[Vladimir Oltean]

On Sun, Jun 11, 2023 at 11:15:42AM +0300, Arınç ÜNAL wrote:
> The CPU_PORT bits represent the CPU port to trap frames to for the MT7530
> switch. This switch traps frames to the CPU port set on the CPU_PORT bits,
> regardless of the affinity of the user port which the frames are received
> from.
> 
> When multiple CPU ports are being used, the trapped frames won't be
> received when the DSA conduit interface, which the frames are supposed to
> be trapped to, is down because it's not affine to any user port. This
> requires the DSA conduit interface to be manually set up for the trapped
> frames to be received.
> 
> To fix this, implement ds->ops->master_state_change() on this subdriver and
> set the CPU_PORT bits to the CPU port which the DSA conduit interface its
> affine to is up. Introduce the active_cpu_ports field to store the
> information of the active CPU ports. Correct the macros, CPU_PORT is bits 4
> through 6 of the register.
> 
> Add comments to explain frame trapping for this switch.
> 
> Fixes: b8f126a8d543 ("net-next: dsa: add dsa support for Mediatek MT7530 switch")
> Suggested-by: Vladimir Oltean <olteanv at gmail.com>
> Signed-off-by: Arınç ÜNAL <arinc.unal at arinc9.com>
> ---

My only concern with this patch is that it depends upon functionality
that was introduced in kernel v5.18 - commit 295ab96f478d ("net: dsa:
provide switch operations for tracking the master state"). But otherwise
it is correct, does not require subsequent net-next rework, and
relatively clean, at least I think it's cleaner than checking which of
the multiple CPU ports is the active CPU port - the other will have no
user port dp->cpu_dp pointing to it. But strictly, the master_state_change()
logic is not needed when you can't change the CPU port assignment.

It might also be that your patch "net: dsa: introduce
preferred_default_local_cpu_port and use on MT7530" gets backported
to stable kernels that this patch doesn't get backported to, and then,
we have a problem, because that will cause even more breakage.

I wonder if there's a way to specify a dependency from this to that
other patch, to ensure that at least that does not happen?