linux-kernel - Re: [PATCH net-next v3 1/7] net: dsa: mt7530: empty default case on mt7530_setup

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Date: Fri, 2 Feb 2024 21:25:55 +0100
From: Andrew Lunn <andrew@...n.ch>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Arınç ÜNAL <arinc.unal@...nc9.com>,
	Daniel Golle <daniel@...rotopia.org>,
	DENG Qingfang <dqfext@...il.com>,
	Sean Wang <sean.wang@...iatek.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	Vladimir Oltean <olteanv@...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>,
	mithat.guner@...ont.com, erkin.bozoglu@...ont.com,
	Bartel Eerdekens <bartel.eerdekens@...stell8.be>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH net-next v3 1/7] net: dsa: mt7530: empty default case on
 mt7530_setup_port5()

On Fri, Feb 02, 2024 at 06:05:36PM +0000, Russell King (Oracle) wrote:
> On Fri, Feb 02, 2024 at 08:44:39PM +0300, Arınç ÜNAL wrote:
> > On 2.02.2024 14:40, Russell King (Oracle) wrote:
> > > While reviewing this change, but not related to it, I notice that this
> > > function sets the TX delay based on the RGMII interface mode. This isn't
> > > correct. I've explained why this is this many times in the past, but
> > > essentially it comes down to the model:
> > > 
> > > 
> > > phy-mode in NIC node	Network driver	PCB		PHY
> > > rgmii			no delays	delays		no delays
> > > rgmii-id		no delays	no delays	tx/rx delays
> > > rgmii-txid		no delays	no delays	tx delays
> > > rgmii-rxid		no delays	no delays	rx delays
> > > 
> > > Then we have rx-internal-delay-ps and tx-internal-delay-ps in the NIC
> > > node which define the RGMII delays at the local end and similar
> > > properties for the PHY node.
> > > 
> > > 
> > > So, if we take the view that, when a switch is connected to a NIC in
> > > RGMII mode, then the phy-mode specified delays still should not impact
> > > the local NIC.
> > > 
> > > Now, for the switch, we specify the phy-mode in the port node as well.
> > > Consider the case of a switch port connected to a RGMII PHY. This has
> > > to operate in exactly the same way as a normal NIC - that is, the
> > > RGMII delays at the port should be ignored as it's the responsibility
> > > of a PHY.

Not quite. It is unusual, but there are a few MAC drivers which do act
on phy-mode, to configure MAC delays. However, if they do this, they
then mask the value of phy-mode passed to the PHY in order to avoid
double delays. Its not something i recommend, we prefer the PHYs do
the delays. And it something i strongly suggest we avoid in DSA.

> > > The final scenario to examine is the case of a RGMII switch port
> > > connected to a NIC.

This should also extend to a port connected to a port of another
switch. For Marvell switches, these are so called DSA ports.

> > > The NIC's phy-mode has no way to be communicated
> > > to DSA or vice versa, so neither phy-mode can impact the other side
> > > of the RGMII link, but should only place the link into RGMII mode.
> > > Given everything I've said above, the only way to configure RGMII
> > > delays is via the rx-internal-delay-ps and tx-internal-delay-ps
> > > properties. So, DSA drivers should _not_ be configuring their ports
> > > with RGMII delays based on the RGMII phy interface mode.

Marvell goes against this. rx-internal-delay-ps and
tx-internal-delay-ps are pretty new, when compared to the age of
mv88e6xxx. I had the problem of a FEC connected to an mv88e6xxx using
RGMII and i needed to somehow configure RGMII delays, or packets did
not get through. So mv88e6xxx will apply phy-mode to ports being used
in CPU or DSA mode. So in the case of the FEC->switch, rgmii-id is
used by the switch. For DSA->DSA, there are DT blobs with rgmii-txid
on both ends of the link, which results in the needed delays.

> > > The above is my purely logically reasoned point of view on this
> > > subject. Others may have other (to me completely illogical)
> > > interpretations that can only lead to interoperability issues.

Now that rx-internal-delay-ps and tx-internal-delay-ps actually exist,
these are the best ways to handle such delays, for new drivers. But we
cannot change old drivers without causing regressions.

       Andrew