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

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Date: Tue, 13 Jun 2023 18:08:15 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Arınç ÜNAL <arinc9.unal@...il.com>
Cc: Arınç ÜNAL <arinc.unal@...nc9.com>,
	Daniel Golle <daniel@...rotopia.org>,
	Landen Chao <Landen.Chao@...iatek.com>,
	DENG Qingfang <dqfext@...il.com>,
	Sean Wang <sean.wang@...iatek.com>, Andrew Lunn <andrew@...n.ch>,
	Florian Fainelli <f.fainelli@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	Russell King <linux@...linux.org.uk>,
	Frank Wunderlich <frank-w@...lic-files.de>,
	Bartel Eerdekens <bartel.eerdekens@...stell8.be>,
	mithat.guner@...ont.com, erkin.bozoglu@...ont.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH net v2 2/7] net: dsa: mt7530: fix trapping frames with
 multiple CPU ports on MT7530

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@...il.com>
> Signed-off-by: Arınç ÜNAL <arinc.unal@...nc9.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?